🔧 [5] Operators & Control Flow — Decision Making in Java!

✨ Introduction

Ready to make your code a little smarter? 🚀 Once you've got a handle on variables and data types, the next big leap is teaching your program how to think, decide, and act.

That's where operators and control flow come in. Whether it's doing some quick math 🧮, checking conditions 🤔, or repeating tasks 🔁 — these tools help your program react to the world like a mini decision-maker.

In this guide, you'll explore Java's powerful tools like arithmetic, logical, and comparison operators. Then we'll dive into decision-making structures like if and switch, and loop through tasks using for, while, and do-while loops.

By the end, you'll have the building blocks to control your program's flow and bring logic to life! 💡

➕ Java Operators Overview

🧮 Arithmetic Operators

These perform basic math operations.

Operator	Description	Example
+	Addition	a + b
-	Subtraction	a - b
*	Multiplication	a * b
/	Division	a / b
%	Remainder (Modulus)	a % b

🔎 Relational (Comparison) Operators

These operators are used in conditions to compare values.

Operator	Description	Example
==	Equal to	a == b
!=	Not equal to	a != b
>	Greater than	a > b
<	Less than	a < b
>=	Greater than or equal to	a >= b
<=	Less than or equal to	a <= b

🧠 Logical Operators

These operators combine multiple conditions together.

Operator	Description	Example
&&	AND (both conditions must be true)	a > 5 && b < 10
||	OR (at least one condition is true)	a > 5 || b < 10
!	NOT (reverses the condition)	!isReady

🧪 Examples of Operators — Building Logic

🧮 Arithmetic Operators

int a = 12, b = 4;

System.out.println(a + b); // ➕ 16 (Addition)
System.out.println(a - b); // ➖ 8  (Subtraction)
System.out.println(a * b); // ✖️ 48 (Multiplication)
System.out.println(a / b); // ➗ 3  (Division)
System.out.println(a % b); // 🔁 0  (Remainder)

🔎 Relational (Comparison) Operators

int x = 7, y = 10;

System.out.println(x == y);  // ❓ false (7 is not equal to 10)
System.out.println(x != y);  // ✅ true  (7 is not equal to 10)
System.out.println(x > y);   // ❌ false (7 is not greater than 10)
System.out.println(x < y);   // ✅ true  (7 is less than 10)
System.out.println(x <= 7);  // ✅ true  (7 is less than or equal to 7)

🧠 Logical Operators

boolean isAdult = true;
boolean hasTicket = false;
int score = 85;

System.out.println(isAdult && hasTicket);         // ❌ false (only one is true)
System.out.println(isAdult || hasTicket);         // ✅ true (at least one is true)
System.out.println(!(score > 50));                // ❌ false (score is greater than 50, negated)
System.out.println(score > 80 && score < 100);    // ✅ true (both conditions met)
System.out.println(score < 50 || score > 90);     // ❌ false (neither condition is true)

🧩 Practice Questions — Try It Yourself

🧮 Arithmetic Operator Questions

// Q1: Add two numbers: 12 and 8
// Q2: Subtract 5 from 20
// Q3: Multiply 7 and 6
// Q4: Divide 100 by 25
// Q5: Find the remainder when 22 is divided by 7

// Hint: Use +, -, *, /, % operators respectively

🔎 Relational Operator Questions

// Q1: Check if 10 is equal to 10
// Q2: Check if 15 is not equal to 20
// Q3: Is 100 greater than 99?
// Q4: Is 25 less than 15?
// Q5: Is 30 greater than or equal to 30?

// Hint: Use ==, !=, >, <, >=, <=

🧠 Logical Operator Questions

// Q1: Check if a person is an adult (age >= 18) AND has a valid ID
// Q2: Check if a student passed (marks > 40) OR has grace marks
// Q3: Reverse the condition: "isRaining"
// Q4: Check if temperature > 20 AND < 35
// Q5: Check if a number is either less than 0 OR greater than 100

// Hint: Use &&, ||, ! for combining conditions

📝 Answers with Explanations

🧮 Arithmetic Operator Answers

// Q1
System.out.println(12 + 8); // ➕ 20
// ✅ Explanation: Adds two integers.
// ❌ Common Mistake: Using strings instead: "12" + "8" gives "128"

// Q2
System.out.println(20 - 5); // ➖ 15
// ✅ Explanation: Subtracts 5 from 20.
// ❌ Common Mistake: Reversing the operands: 5 - 20 = -15

// Q3
System.out.println(7 * 6); // ✖️ 42
// ✅ Explanation: Multiplies 7 and 6.
// ❌ Common Mistake: Typing x instead of *

// Q4
System.out.println(100 / 25); // ➗ 4
// ✅ Explanation: Integer division; no decimals here.
// ❌ Common Mistake: Using 0 as divisor (runtime error)

// Q5
System.out.println(22 % 7); // 🔁 1
// ✅ Explanation: 7 fits in 22 three times (21), remainder is 1.
// ❌ Common Mistake: Confusing modulus with division

🔎 Relational Operator Answers

// Q1
System.out.println(10 == 10); // ✅ true
// ✅ Explanation: Both sides are equal.
// ❌ Common Mistake: Using = instead of ==

// Q2
System.out.println(15 != 20); // ✅ true
// ✅ Explanation: 15 is not equal to 20.
// ❌ Common Mistake: Reversing != to == by habit

// Q3
System.out.println(100 > 99); // ✅ true
// ✅ Explanation: 100 is greater than 99.
// ❌ Common Mistake: Using < accidentally

// Q4
System.out.println(25 < 15); // ✅ false
// ✅ Explanation: 25 is not less than 15.
// ❌ Common Mistake: Reading it backwards

// Q5
System.out.println(30 >= 30); // ✅ true
// ✅ Explanation: 30 is equal to 30, so ≥ is true.
// ❌ Common Mistake: Confusing >= with >

🧠 Logical Operator Answers

// Q1
int age = 20;
boolean hasID = true;
System.out.println(age >= 18 && hasID); // ✅ true
// ✅ Explanation: Both conditions are true
// ❌ Common Mistake: Using & instead of &&

// Q2
int marks = 35;
boolean grace = true;
System.out.println(marks > 40 || grace); // ✅ true
// ✅ Explanation: One condition is true (grace)
// ❌ Common Mistake: Using && instead of ||

// Q3
boolean isRaining = true;
System.out.println(!isRaining); // ✅ false
// ✅ Explanation: ! reverses true to false
// ❌ Common Mistake: Writing ! without parentheses or variable

// Q4
int temp = 25;
System.out.println(temp > 20 && temp < 35); // ✅ true
// ✅ Explanation: 25 is in the range 21-34
// ❌ Common Mistake: Using || instead of &&

// Q5
int number = -5;
System.out.println(number < 0 || number > 100); // ✅ true
// ✅ Explanation: One condition is true (number < 0)
// ❌ Common Mistake: Not grouping conditions properly

💡 Tip: Practice modifying the values above and observe how the output changes — this is the best way to master logic!

🧩 Word Problems: Build DSA Thinking Using Java Operators

🧮 Arithmetic Operators — 5 Problems

1. Shopping Cart Total: You bought 3 notebooks for ₹40 each and 2 pens for ₹15 each. Find the total bill amount.
2. Average Marks: Your scores in 3 subjects are 78, 85, and 92. Find the average score.
3. Area of Rectangle: Given length = 15 units and width = 8 units, calculate the area.
4. Splitting Bill: You have ₹1,000 and want to divide it equally among 4 friends. How much will each get?
5. Leftover Chocolates: You bought 53 chocolates. If each kid gets 6, how many are left after distribution?

🔎 Relational Operators — 5 Problems

1. Age Eligibility: Check if a person aged 17 is eligible to vote (must be 18 or older).
2. Exam Pass Check: A student scores 43 marks. Pass mark is 40. Did they pass?
3. Speeding Fine: A car is going at 95 km/h in a 90 km/h zone. Should it get fined?
4. Same Numbers: Check if two entered numbers are exactly the same.
5. Budget Check: You want to buy a phone for ₹25,000. Your budget is ₹20,000. Is it affordable?

🧠 Logical Operators — 5 Problems

1. Exam Criteria: You passed both theory (≥40) and practical (≥35). Check if you passed overall.
2. Discount Eligibility: You're eligible for a discount if you're a student or a senior citizen. Write logic for that.
3. Temperature Range: Check if temperature is between 20°C and 30°C (inclusive).
4. Weekend Checker: Today is Saturday. Write a condition that checks if today is Saturday or Sunday.
5. Security Check: You're not allowed if you're on the banned list. Write a NOT condition to allow entry if not banned.

✅ Answers: Word Problems on Operators

🧮 Arithmetic Operator Answers

// Q1
int total = (3 * 40) + (2 * 15);
System.out.println("Total Bill: ₹" + total); // ₹150

// Q2
int avg = (78 + 85 + 92) / 3;
System.out.println("Average Marks: " + avg); // 85

// Q3
int area = 15 * 8;
System.out.println("Area: " + area); // 120

// Q4
int eachGets = 1000 / 4;
System.out.println("Each gets: ₹" + eachGets); // ₹250

// Q5
int leftover = 53 % 6;
System.out.println("Leftover chocolates: " + leftover); // 5

🔎 Relational Operator Answers

// Q1
int age = 17;
System.out.println(age >= 18); // false

// Q2
int marks = 43;
System.out.println(marks >= 40); // true

// Q3
int speed = 95;
System.out.println(speed > 90); // true

// Q4
int a = 10, b = 10;
System.out.println(a == b); // true

// Q5
int price = 25000, budget = 20000;
System.out.println(price <= budget); // false

🧠 Logical Operator Answers

// Q1
int theory = 45, practical = 37;
System.out.println(theory >= 40 && practical >= 35); // true

// Q2
boolean isStudent = true;
boolean isSenior = false;
System.out.println(isStudent || isSenior); // true

// Q3
int temp = 25;
System.out.println(temp >= 20 && temp <= 30); // true

// Q4
String day = "Saturday";
System.out.println(day.equals("Saturday") || day.equals("Sunday")); // true

// Q5
boolean isBanned = false;
System.out.println(!isBanned); // true (entry allowed)

🧠 15 Arithmetic Challenger Problems

1. Train Distance: A train travels at 60 km/h. How far will it go in 5.5 hours?
2. Currency Converter: Convert ₹500 to USD (Assume 1 USD = ₹82).
3. Square Perimeter: Find the perimeter of a square with side length 19 cm.
4. Apple Distribution: You have 365 apples. Distribute equally among 12 baskets. How many does each get?
5. Mobile Data Usage: You have 1.5GB per day for 30 days. What's the total monthly data?
6. Library Fine: ₹2 fine/day. You returned a book 18 days late. Total fine?
7. Bill Split with Tip: Your total bill is ₹750. Tip is 10%. Split among 5 friends.
8. Simple Interest: ₹1000 at 5% annual interest for 2 years.
9. Discount Price: Original price is ₹800. Discount = 15%. What's the new price?
10. Circle Area: Radius is 7 cm. Use 3.14 as π.
11. Temperature Convert: Convert 100°C to Fahrenheit (F = C × 9/5 + 32).
12. Electricity Bill: Unit rate = ₹6.5. Total units = 175. Total bill?
13. Marks Percentage: You scored 540/600. What's your percentage?
14. Speed Calculation: You ran 400m in 50 seconds. What's the speed in m/s?
15. Time Taken: Distance = 900 km, Speed = 60 km/h. Time taken?

🧮 Arithmetic Operator Answers

public class ArithmeticChallengerAnswers {
    public static void main(String[] args) {
        // Q1: Train Distance
        double distance = 60 * 5.5;
        System.out.println("Q1 - Distance Travelled: " + distance + " km"); // 330.0 km

        // Q2: Currency Converter
        double usd = 500.0 / 82;
        System.out.println("Q2 - USD Amount: $" + usd); // ~$6.10

        // Q3: Square Perimeter
        int perimeter = 4 * 19;
        System.out.println("Q3 - Perimeter: " + perimeter + " cm"); // 76 cm

        // Q4: Apple Distribution
        int applesEach = 365 / 12;
        System.out.println("Q4 - Apples per basket: " + applesEach); // 30

        // Q5: Mobile Data Usage
        double totalData = 1.5 * 30;
        System.out.println("Q5 - Total Monthly Data: " + totalData + " GB"); // 45.0 GB

        // Q6: Library Fine
        int fine = 2 * 18;
        System.out.println("Q6 - Total Fine: ₹" + fine); // ₹36

        // Q7: Bill Split with Tip
        double totalWithTip = 750 + (0.10 * 750);
        double perPerson = totalWithTip / 5;
        System.out.println("Q7 - Each Pays: ₹" + perPerson); // ₹165.0

        // Q8: Simple Interest
        // Formula:
        // SI = (P × R × T) / 100
        // Where:
        // P = Principal amount
        // R = Rate of interest (annual)
        // T = Time in years

        double si = (1000 * 5 * 2) / 100.0;
        System.out.println("Q8 - Simple Interest: ₹" + si); // ₹100.0

        // Q9: Discount Price
        double discount = 0.15 * 800;
        double newPrice = 800 - discount;
        System.out.println("Q9 - Discounted Price: ₹" + newPrice); // ₹680.0

        // Q10: Circle Area
        // Formula:
        // A = π × r²
        // Where:
        // A = Area of the circle
        // π = Pi (approximately 3.1416)
        // r = Radius of the circle

        double area = 3.14 * 7 * 7;
        System.out.println("Q10 - Circle Area: " + area + " cm²"); // 153.86 cm²

        // Q11: Temperature Convert
        // Temperature Conversion: Celsius to Fahrenheit
        // Formula:
        // F = (C × 9/5) + 32
        // Where:
        // C = Temperature in Celsius
        // F = Temperature in Fahrenheit

        double fahrenheit = (100 * 9.0/5.0) + 32;
        System.out.println("Q11 - Fahrenheit: " + fahrenheit + "°F"); // 212.0°F

        // Q12: Electricity Bill
        double bill = 6.5 * 175;
        System.out.println("Q12 - Electricity Bill: ₹" + bill); // ₹1137.5

        // Q13: Marks Percentage
        double percent = (540.0 / 600.0) * 100;
        System.out.println("Q13 - Percentage: " + percent + "%"); // 90.0%

        // Q14: Speed Calculation
        double speed = 400.0 / 50.0;
        System.out.println("Q14 - Speed: " + speed + " m/s"); // 8.0 m/s

        // Q15: Time Taken
        double time = 900.0 / 60.0;
        System.out.println("Q15 - Time Taken: " + time + " hours"); // 15.0 hours
    }
}

🔎 15 Relational Challenger Problems

1. Eligibility Check: Check if someone who is 12 is a teenager (13-19).
2. Perfect Score: Check if a student's score is exactly 100.
3. Profit or Loss: Selling price = ₹950, cost price = ₹1000. Was it a loss?
4. Equal Sides: Check if a triangle with sides 5, 5, 5 is equilateral.
5. Even Age: Check if an age is even (divisible by 2).
6. Greater Amount: Compare two bank balances: ₹15,400 and ₹16,000.
7. Top Rank: A student scores the highest in class. Check if score equals max score.
8. Loan Limit: User requests ₹50,000. Max allowed is ₹40,000. Approve?
9. Rectangle or Square: Check if length and breadth are equal.
10. Temperature Comparison: Is today hotter than yesterday?
11. Speed Violation: Check if car exceeds 80 km/h speed limit.
12. Correct Pin: Verify if entered PIN matches the correct one.
13. Attendance Requirement: Is 72% attendance enough for required 75%?
14. Minimum Age: Check if user is at least 21 to rent a car.
15. Three Equal Numbers: Are all 3 entered numbers the same?

🔎 Relational Operator Answers

public class RelationalChallengerAnswers {
    public static void main(String[] args) {
        // Q1: Eligibility Check (Teenager: age 13-19)
        int age = 12;
        boolean isTeenager = age >= 13 && age <= 19;
        System.out.println("Q1 - Is Teenager? " + isTeenager); // false

        // Q2: Perfect Score
        int score = 100;
        boolean isPerfect = score == 100;
        System.out.println("Q2 - Is Perfect Score? " + isPerfect); // true

        // Q3: Profit or Loss
        int sp = 950, cp = 1000;
        boolean isLoss = sp < cp;
        System.out.println("Q3 - Was it a Loss? " + isLoss); // true

        // Q4: Equal Sides
        int s1 = 5, s2 = 5, s3 = 5;
        boolean isEquilateral = (s1 == s2) && (s2 == s3);
        System.out.println("Q4 - Is Equilateral? " + isEquilateral); // true

        // Q5: Even Age
        int ageCheck = 24;
        boolean isEven = ageCheck % 2 == 0;
        System.out.println("Q5 - Is Even Age? " + isEven); // true

        // Q6: Greater Amount
        int balance1 = 15400, balance2 = 16000;
        boolean isSecondGreater = balance2 > balance1;
        System.out.println("Q6 - Is ₹16000 greater than ₹15400? " + isSecondGreater); // true

        // Q7: Top Rank
        int studentScore = 98, maxScore = 98;
        boolean isTopRanker = studentScore == maxScore;
        System.out.println("Q7 - Is Top Ranker? " + isTopRanker); // true

        // Q8: Loan Limit
        int requested = 50000, maxAllowed = 40000;
        boolean isApproved = requested <= maxAllowed;
        System.out.println("Q8 - Loan Approved? " + isApproved); // false

        // Q9: Rectangle or Square
        int length = 10, breadth = 10;
        boolean isSquare = length == breadth;
        System.out.println("Q9 - Is Square? " + isSquare); // true

        // Q10: Temperature Comparison
        int todayTemp = 37, yesterdayTemp = 35;
        boolean isHotter = todayTemp > yesterdayTemp;
        System.out.println("Q10 - Is Today Hotter? " + isHotter); // true

        // Q11: Speed Violation
        int speed = 85;
        boolean isOverSpeeding = speed > 80;
        System.out.println("Q11 - Speeding Violation? " + isOverSpeeding); // true

        // Q12: Correct Pin
        int enteredPin = 1234, correctPin = 1234;
        boolean isPinCorrect = enteredPin == correctPin;
        System.out.println("Q12 - Is Pin Correct? " + isPinCorrect); // true

        // Q13: Attendance Requirement
        double attendance = 72.0, required = 75.0;
        boolean meetsRequirement = attendance >= required;
        System.out.println("Q13 - Meets Attendance Requirement? " + meetsRequirement); // false

        // Q14: Minimum Age
        int userAge = 21;
        boolean canRentCar = userAge >= 21;
        System.out.println("Q14 - Can Rent Car? " + canRentCar); // true

        // Q15: Three Equal Numbers
        int a = 7, b = 7, c = 7;
        boolean allEqual = (a == b) && (b == c);
        System.out.println("Q15 - Are All Numbers Equal? " + allEqual); // true
    }
}

🧠 15 Logical Challenger Problems

1. Voter Eligibility: Check if person is Indian and 18+ to vote.
2. Scholarship Criteria: Marks ≥ 90 or family income < ₹2,00,000.
3. Loan Approval: Age ≥ 25 and salary ≥ ₹30,000.
4. Attendance Alert: Attendance < 75% or marks < 40 → Show warning?
5. Exam Cheating Detector: isCheating = true → Block exam using !.
6. Working Hours: Check if employee worked between 9AM and 6PM.
7. Valid User: User must have username and password to log in.
8. Age or Membership: Entry allowed if age ≥ 60 or isPremiumMember.
9. Not Empty String: isEmpty = false → Allow form submit?
10. Account Active: Active user and email verified → Allow access.
11. Shopping Deal: Add to cart if product in stock and discount ≥ 10%.
12. Logical Puzzle: (a > b) and (b > c) → is a > c?
13. Event Entry: Age ≥ 18 and hasTicket or isGuest.
14. Security Alert: !isVerified → Show warning popup.
15. Salary Range: Is salary between ₹25,000 and ₹50,000?

🧠 Logical Operator Answers

public class LogicalChallengerAnswers {
    public static void main(String[] args) {
        // Q1: Voter Eligibility
        boolean isIndian = true;
        int age = 20;
        boolean canVote = isIndian && age >= 18;
        System.out.println("Q1 - Can Vote? " + canVote); // true

        // Q2: Scholarship Criteria
        int marks = 85;
        int income = 180000;
        boolean getsScholarship = marks >= 90 || income < 200000;
        System.out.println("Q2 - Eligible for Scholarship? " + getsScholarship); // true

        // Q3: Loan Approval
        int personAge = 28;
        int salary = 32000;
        boolean loanApproved = personAge >= 25 && salary >= 30000;
        System.out.println("Q3 - Loan Approved? " + loanApproved); // true

        // Q4: Attendance Alert
        double attendance = 70.0;
        double testMarks = 38.0;
        boolean showWarning = attendance < 75 || testMarks < 40;
        System.out.println("Q4 - Show Warning? " + showWarning); // true

        // Q5: Exam Cheating Detector
        boolean isCheating = true;
        boolean allowExam = !isCheating;
        System.out.println("Q5 - Allow Exam? " + allowExam); // false

        // Q6: Working Hours

        // Define the hour someone checks in
        int checkInHour = 10; // This means the person checked in at 10 AM
        
        // Define the working hours using a logical AND (&&) condition
        // Check if the check-in time is between 9 AM and 6 PM (inclusive)
        // This means the time must be greater than or equal to 9 AND less than or equal to 18
        boolean isWorkingHours = checkInHour >= 9 && checkInHour <= 18;
        
        // Explanation:
        // - checkInHour >= 9     → Is 10 greater than or equal to 9? ✅ Yes
        // - checkInHour <= 18    → Is 10 less than or equal to 18? ✅ Yes
        // - Since both conditions are true, the result is true
        
        System.out.println("Q6 - Within Working Hours? " + isWorkingHours); // Output: true

        // Q7: Valid User
        boolean hasUsername = true;
        boolean hasPassword = true;
        boolean canLogin = hasUsername && hasPassword;
        System.out.println("Q7 - Can Login? " + canLogin); // true

        // Q8: Age or Membership
        int userAge = 45;
        boolean isPremiumMember = true;
        boolean allowEntry = userAge >= 60 || isPremiumMember;
        System.out.println("Q8 - Entry Allowed? " + allowEntry); // true

        // Q9: Not Empty String
        boolean isEmpty = false;
        boolean canSubmit = !isEmpty;
        System.out.println("Q9 - Allow Form Submit? " + canSubmit); // true

        // Q10: Account Active
        boolean isActive = true;
        boolean isEmailVerified = true;
        boolean allowAccess = isActive && isEmailVerified;
        System.out.println("Q10 - Access Granted? " + allowAccess); // true

        // Q11: Shopping Deal
        boolean inStock = true;
        int discount = 15;
        boolean addToCart = inStock && discount >= 10;
        System.out.println("Q11 - Add to Cart? " + addToCart); // true

        // Q12: Logical Puzzle

        int a = 10, b = 8, c = 5;
        
        // We are checking three comparisons:
        // 1. a > b → Is 'a' greater than 'b'? → true
        // 2. b > c → Is 'b' greater than 'c'? → true
        // 3. a > c → Is 'a' greater than 'c'? → true

        // ✅ In mathematics, if (a > b) and (b > c), we can conclude (a > c).
        //    This is called a *transitive relation*.

        // ✅ So in theory, we don't need to check (a > c) separately —
        //    it will always be true if the first two are true.

        // 🛡️ BUT in real-world code, values may change,
        //    and logic may evolve — so including (a > c) as an explicit condition:
        //    - Makes your logic clearer to other programmers
        //    - Acts as a safety check in case 'b' gets changed unexpectedly
        //    - Helps avoid bugs from assuming transitivity in changing contexts
        
        boolean isAGreaterThanC = (a > b) && (b > c) && (a > c);
        
        System.out.println("Q12 - Is a > c? " + isAGreaterThanC); // Output: true

        // Q13: Event Entry
        int guestAge = 17;
        boolean hasTicket = false;
        boolean isGuest = true;
        boolean canEnter = (guestAge >= 18 && hasTicket) || isGuest;
        System.out.println("Q13 - Can Enter Event? " + canEnter); // true

        // Q14: Security Alert
        boolean isVerified = false;
        boolean showPopup = !isVerified;
        System.out.println("Q14 - Show Security Alert? " + showPopup); // true

        // Q15: Salary Range
        int empSalary = 42000;
        boolean isInRange = empSalary >= 25000 && empSalary <= 50000;
        System.out.println("Q15 - Salary in Range? " + isInRange); // true
    }
}

🔀 Control Flow Statements

✅ if / else if / else

Use this when you want your program to make decisions based on certain conditions.

int number = 7;

if (number > 0) {
    System.out.println("Positive");
} else if (number < 0) {
    System.out.println("Negative");
} else {
    System.out.println("Zero");
}

✅ Explanation: The program checks if the number is positive, negative, or zero and prints the appropriate result.

❌ Expected Mistake: Writing if (number = 0) instead of if (number == 0). The first one assigns 0 instead of comparing it.

🌀 switch Statement

Use switch when you have multiple exact values to check against a single variable.

int day = 3;

switch (day) {
    case 1: System.out.println("Monday"); break;
    case 2: System.out.println("Tuesday"); break;
    case 3: System.out.println("Wednesday"); break;
    default: System.out.println("Invalid Day");
}

✅ Explanation: This prints the day name based on the number. The break is important to stop the flow after each match.

❌ Expected Mistake: Omitting break will cause multiple cases to execute (called "fall-through").

🔁 Loops — Repeat Until You're Done

➡️ for Loop

Best when you know how many times to repeat something. It has three parts: initialization, condition, and update.

for (int i = 1; i <= 5; i++) {
    System.out.println("Count: " + i);
}

✅ Explanation: This prints numbers 1 to 5. Loop starts at 1 and runs while i <= 5.

❌ Expected Mistake: Accidentally putting i++ in the condition (like i++ <= 5), which changes value before the check.

🔁 while Loop

Use when you don't know how many times the loop should run — the condition is checked before each run.

int i = 1;
while (i <= 3) {
    System.out.println("Looping: " + i);
    i++;
}

✅ Explanation: Repeats the loop until i becomes greater than 3.

❌ Expected Mistake: Forgetting i++ causes an infinite loop.

🔂 do-while Loop

This loop runs at least once, even if the condition is false.

int i = 1;
do {
    System.out.println("Running: " + i);
    i++;
} while (i <= 3);

✅ Explanation: The code inside do runs once before the condition is even checked.

❌ Expected Mistake (Misunderstanding): A beginner might think: "If the condition is false from the beginning, the code inside should never run."

🔴 But that would be true for a while loop — not for a do-while. In a do-while loop, the block of code runs at least once before the condition is checked. So even if the condition is false initially, the code still executes once.

🚫 Common Mistake in Loop Understanding:

Concept	Mistake
do-while	Assuming the condition is checked before running the loop
Truth	The loop body always runs at least once, no matter the condition
Common Error	Thinking it behaves like a while loop (which doesn't run at all if false)

🎯 15 Best if, else if, and else Concept Challenges

1. Grade Checker: Write a program that takes student marks as input and prints:
   • "A" for marks ≥ 90
   • "B" for marks between 80 and 89
   • "C" for marks between 70 and 79
   • Else, print "Fail"
2. Temperature Advice: Given a temperature value, print:
   • "Too Cold" if temperature < 10
   • "Moderate" if between 10 and 25
   • "Hot" if above 25
3. Even/Odd Check: Given a number, check whether it's even or odd and print "Even" or "Odd".
4. Age Category: Given a person's age, print:
   • "Child" for ages 0–12
   • "Teen" for 13–19
   • "Adult" for 20–59
   • "Senior" for 60 and above
5. Login Status: If a user is not logged in, print "Login Required"; else, print "Welcome back".
6. Number Sign: Given a number, print:
   • "Positive" if the number is greater than 0
   • "Zero" if the number is 0
   • "Negative" if less than 0
7. Day Checker: Given a number (1–7), print the corresponding weekday:
   • 1 = Monday, 2 = Tuesday, ..., 7 = Sunday
   • Any other number should print "Not a valid day"
8. BMI Category: Given the BMI value, print:
   • "Underweight" if BMI < 18.5
   • "Normal" if between 18.5 and 24.9
   • "Overweight" if between 25 and 29.9
   • "Obese" if BMI ≥ 30
9. Exam Result: Based on marks:
   • ≥90 → "Distinction"
   • 60–89 → "Passed"
   • <60 → "Failed"
10. Traffic Light: Based on traffic signal color (red/yellow/green), print:
    • "Stop" for "red"
    • "Ready" for "yellow"
    • "Go" for "green"
    • Any other value → "Invalid Signal"
11. Movie Rating: Based on rating:
    • ≥4.5 → "Excellent"
    • 3.5–4.4 → "Good"
    • 2.5–3.4 → "Average"
    • Else → "Poor"
12. Password Strength: Based on password length:
    • ≥12 characters → "Strong"
    • 8–11 characters → "Moderate"
    • <8 characters → "Weak"
13. Bill Discount: Based on total bill:
    • ≥5000 → "20% Discount"
    • 3000–4999 → "10% Discount"
    • Else → "No Discount"
14. Voting Zone: Based on city:
    • "Delhi" → "Zone A"
    • "Mumbai" → "Zone B"
    • Else → "Zone C"
15. Student Category: Based on age:
    • 0–5 → "Toddler"
    • 6–12 → "School"
    • 13–18 → "High School"
    • 19+ → "College Student"

🔀 if, else if, and else Challenge Solutions

public class IfElseChallengerAnswers {
    public static void main(String[] args) {

        // Q1: Grade Checker
        int marks = 85;
        if (marks >= 90) {
            System.out.println("Q1 - Grade: A");
        } else if (marks >= 80) {
            System.out.println("Q1 - Grade: B");
        } else if (marks >= 70) {
            System.out.println("Q1 - Grade: C");
        } else {
            System.out.println("Q1 - Grade: Fail");
        }

        // Q2: Temperature Advice
        int temp = 15;
        if (temp < 10) {
            System.out.println("Q2 - Too Cold");
        } else if (temp <= 25) {
            System.out.println("Q2 - Moderate");
        } else {
            System.out.println("Q2 - Hot");
        }

        // Q3: Even/Odd Check
        int num = 13;
        if (num % 2 == 0) {
            System.out.println("Q3 - Even");
        } else {
            System.out.println("Q3 - Odd");
        }

        // Q4: Age Category
        int age = 65;
        if (age <= 12) {
            System.out.println("Q4 - Child");
        } else if (age <= 19) {
            System.out.println("Q4 - Teen");
        } else if (age <= 59) {
            System.out.println("Q4 - Adult");
        } else {
            System.out.println("Q4 - Senior");
        }

        // Q5: Login Status
        boolean isLoggedIn = false;
        if (!isLoggedIn) {
            System.out.println("Q5 - Login Required");
        } else {
            System.out.println("Q5 - Welcome back");
        }

        // Q6: Number Sign
        int value = 0;
        if (value > 0) {
            System.out.println("Q6 - Positive");
        } else if (value == 0) {
            System.out.println("Q6 - Zero");
        } else {
            System.out.println("Q6 - Negative");
        }

        // Q7: Day Checker
        int day = 3;
        if (day == 1) {
            System.out.println("Q7 - Monday");
        } else if (day == 2) {
            System.out.println("Q7 - Tuesday");
        } else if (day == 3) {
            System.out.println("Q7 - Wednesday");
        } else if (day == 4) {
            System.out.println("Q7 - Thursday");
        } else if (day == 5) {
            System.out.println("Q7 - Friday");
        } else if (day == 6) {
            System.out.println("Q7 - Saturday");
        } else if (day == 7) {
            System.out.println("Q7 - Sunday");
        } else {
            System.out.println("Q7 - Not a valid day");
        }

        // Q8: BMI Category
        double bmi = 27.5;
        if (bmi < 18.5) {
            System.out.println("Q8 - Underweight");
        } else if (bmi < 25) {
            System.out.println("Q8 - Normal");
        } else if (bmi < 30) {
            System.out.println("Q8 - Overweight");
        } else {
            System.out.println("Q8 - Obese");
        }

        // Q9: Exam Result
        int studentMarks = 68;
        if (studentMarks >= 90) {
            System.out.println("Q9 - Distinction");
        } else if (studentMarks >= 60) {
            System.out.println("Q9 - Passed");
        } else {
            System.out.println("Q9 - Failed");
        }

        // Q10: Traffic Light
        String light = "yellow";
        if (light.equals("red")) {
            System.out.println("Q10 - Stop");
        } else if (light.equals("yellow")) {
            System.out.println("Q10 - Ready");
        } else if (light.equals("green")) {
            System.out.println("Q10 - Go");
        } else {
            System.out.println("Q10 - Invalid Signal");
        }

        // Q11: Movie Rating
        double rating = 4.7;
        if (rating >= 4.5) {
            System.out.println("Q11 - Excellent");
        } else if (rating >= 3.5) {
            System.out.println("Q11 - Good");
        } else if (rating >= 2.5) {
            System.out.println("Q11 - Average");
        } else {
            System.out.println("Q11 - Poor");
        }

        // Q12: Password Strength
        String password = "hello123";
        if (password.length() >= 12) {
            System.out.println("Q12 - Strong Password");
        } else if (password.length() >= 8) {
            System.out.println("Q12 - Moderate Password");
        } else {
            System.out.println("Q12 - Weak Password");
        }

        // Q13: Bill Discount
        int totalBill = 4500;
        if (totalBill >= 5000) {
            System.out.println("Q13 - 20% Discount");
        } else if (totalBill >= 3000) {
            System.out.println("Q13 - 10% Discount");
        } else {
            System.out.println("Q13 - No Discount");
        }

        // Q14: Voting Zone
        String state = "Delhi";
        if (state.equals("Delhi")) {
            System.out.println("Q14 - Vote in Zone A");
        } else if (state.equals("Mumbai")) {
            System.out.println("Q14 - Vote in Zone B");
        } else {
            System.out.println("Q14 - Vote in Zone C");
        }

        // Q15: Student Category by Age
        int studentAge = 21;
        if (studentAge <= 5) {
            System.out.println("Q15 - Toddler");
        } else if (studentAge <= 12) {
            System.out.println("Q15 - School Student");
        } else if (studentAge <= 18) {
            System.out.println("Q15 - High School");
        } else {
            System.out.println("Q15 - College Student");
        }
    }
}

import java.util.Scanner;

public class IfElseChallengerAnswersInteractive {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        // Q1: Grade Checker
        System.out.print("Q1 - Enter marks: ");
        int marks = sc.nextInt();
        if (marks >= 90) {
            System.out.println("Grade: A");
        } else if (marks >= 80) {
            System.out.println("Grade: B");
        } else if (marks >= 70) {
            System.out.println("Grade: C");
        } else {
            System.out.println("Grade: Fail");
        }

        // Q2: Temperature Advice
        System.out.print("Q2 - Enter temperature: ");
        int temp = sc.nextInt();
        if (temp < 10) {
            System.out.println("Too Cold");
        } else if (temp <= 25) {
            System.out.println("Moderate");
        } else {
            System.out.println("Hot");
        }

        // Q3: Even/Odd Check
        System.out.print("Q3 - Enter a number: ");
        int num = sc.nextInt();
        if (num % 2 == 0) {
            System.out.println("Even");
        } else {
            System.out.println("Odd");
        }

        // Q4: Age Category
        System.out.print("Q4 - Enter age: ");
        int age = sc.nextInt();
        if (age <= 12) {
            System.out.println("Child");
        } else if (age <= 19) {
            System.out.println("Teen");
        } else if (age <= 59) {
            System.out.println("Adult");
        } else {
            System.out.println("Senior");
        }

        // Q5: Login Status
        System.out.print("Q5 - Are you logged in? (true/false): ");
        boolean isLoggedIn = sc.nextBoolean();
        if (!isLoggedIn) {
            System.out.println("Login Required");
        } else {
            System.out.println("Welcome back");
        }

        // Q6: Number Sign
        System.out.print("Q6 - Enter a number: ");
        int value = sc.nextInt();
        if (value > 0) {
            System.out.println("Positive");
        } else if (value == 0) {
            System.out.println("Zero");
        } else {
            System.out.println("Negative");
        }

        // Q7: Day Checker
        System.out.print("Q7 - Enter day number (1-7): ");
        int day = sc.nextInt();
        if (day == 1) {
            System.out.println("Monday");
        } else if (day == 2) {
            System.out.println("Tuesday");
        } else if (day == 3) {
            System.out.println("Wednesday");
        } else if (day == 4) {
            System.out.println("Thursday");
        } else if (day == 5) {
            System.out.println("Friday");
        } else if (day == 6) {
            System.out.println("Saturday");
        } else if (day == 7) {
            System.out.println("Sunday");
        } else {
            System.out.println("Not a valid day");
        }

        // Q8: BMI Category
        System.out.print("Q8 - Enter your BMI: ");
        double bmi = sc.nextDouble();
        if (bmi < 18.5) {
            System.out.println("Underweight");
        } else if (bmi < 25) {
            System.out.println("Normal");
        } else if (bmi < 30) {
            System.out.println("Overweight");
        } else {
            System.out.println("Obese");
        }

        // Q9: Exam Result
        System.out.print("Q9 - Enter exam marks: ");
        int studentMarks = sc.nextInt();
        if (studentMarks >= 90) {
            System.out.println("Distinction");
        } else if (studentMarks >= 60) {
            System.out.println("Passed");
        } else {
            System.out.println("Failed");
        }

        // Q10: Traffic Light
        System.out.print("Q10 - Enter signal (red/yellow/green): ");
        sc.nextLine(); // consume leftover newline
        String light = sc.nextLine();
        if (light.equalsIgnoreCase("red")) {
            System.out.println("Stop");
        } else if (light.equalsIgnoreCase("yellow")) {
            System.out.println("Ready");
        } else if (light.equalsIgnoreCase("green")) {
            System.out.println("Go");
        } else {
            System.out.println("Invalid Signal");
        }

        // Q11: Movie Rating
        System.out.print("Q11 - Enter movie rating (0.0 - 5.0): ");
        double rating = sc.nextDouble();
        if (rating >= 4.5) {
            System.out.println("Excellent");
        } else if (rating >= 3.5) {
            System.out.println("Good");
        } else if (rating >= 2.5) {
            System.out.println("Average");
        } else {
            System.out.println("Poor");
        }

        // Q12: Password Strength
        System.out.print("Q12 - Enter your password: ");
        sc.nextLine(); // consume newline
        String password = sc.nextLine();
        if (password.length() >= 12) {
            System.out.println("Strong Password");
        } else if (password.length() >= 8) {
            System.out.println("Moderate Password");
        } else {
            System.out.println("Weak Password");
        }

        // Q13: Bill Discount
        System.out.print("Q13 - Enter total bill amount: ");
        int totalBill = sc.nextInt();
        if (totalBill >= 5000) {
            System.out.println("20% Discount");
        } else if (totalBill >= 3000) {
            System.out.println("10% Discount");
        } else {
            System.out.println("No Discount");
        }

        // Q14: Voting Zone
        System.out.print("Q14 - Enter your city (Delhi/Mumbai/etc): ");
        sc.nextLine(); // consume newline
        String state = sc.nextLine();
        if (state.equalsIgnoreCase("Delhi")) {
            System.out.println("Vote in Zone A");
        } else if (state.equalsIgnoreCase("Mumbai")) {
            System.out.println("Vote in Zone B");
        } else {
            System.out.println("Vote in Zone C");
        }

        // Q15: Student Category by Age
        System.out.print("Q15 - Enter student age: ");
        int studentAge = sc.nextInt();
        if (studentAge <= 5) {
            System.out.println("Toddler");
        } else if (studentAge <= 12) {
            System.out.println("School Student");
        } else if (studentAge <= 18) {
            System.out.println("High School");
        } else {
            System.out.println("College Student");
        }

        sc.close();
    }
}

🎯 15 Best Switch-Case Concept Challenges

1. Day of the Week: Given an integer from 1 to 7, print the name of the corresponding day (e.g., 1 = Monday, 2 = Tuesday, ..., 7 = Sunday). If the number is outside this range, print "Invalid day".
2. Grade Evaluator: Given a character representing a grade ('A', 'B', 'C', 'D', 'F'), print the corresponding performance (e.g., 'A' = Excellent). If it's any other character, print "Invalid grade".
3. Month Name: Given a number from 1 to 12, print the name of the corresponding month. For any number outside this range, print "Invalid month".
4. Traffic Signal: Given a traffic signal color ("red", "yellow", "green"), print its corresponding action ("Stop", "Wait", "Go"). If the input doesn't match any of these, print "Invalid signal".
5. Calculator Operation: Given two integers and a character representing an operator ('+', '-', '*', '/'), perform the corresponding arithmetic operation. If the operator is invalid, print "Invalid operator". Handle division by zero.
6. Browser Detection: Given a browser name ("Chrome", "Firefox", "Safari"), print "Supported". For any other browser name, print "Not supported".
7. Vowel or Consonant: Given a character (either uppercase or lowercase), check whether it's a vowel ('a', 'e', 'i', 'o', 'u'). If so, print "Vowel". Otherwise, print "Consonant".
8. Level Description: Given a level number (1, 2, or 3), print the difficulty description: 1 = Easy, 2 = Medium, 3 = Hard. If the input is not 1-3, print "Unknown Level".
9. Shape Sides: Given a shape name ("Triangle", "Square", "Pentagon"), print the number of sides. For any other shape, print "Unknown shape".
10. Currency Symbol: Given a currency code ("USD", "EUR", "JPY"), print the corresponding symbol ("$", "€", "¥"). If the code is not recognized, print "Unknown currency".
11. Season Finder: Given a short month name ("Jan", "Feb", ..., "Dec"), print the corresponding season:
    • Winter: "Dec", "Jan", "Feb"
    • Spring: "Mar", "Apr", "May"
    • Summer: "Jun", "Jul", "Aug"
    • Autumn: "Sep", "Oct", "Nov"
    If the month name is invalid, print "Invalid month".
12. Language Greeting: Given a language name ("English", "Spanish", "French"), print a greeting in that language. If it's not one of these, print "Language not supported".
13. Menu Selection: Given a menu option number (1, 2, or 3), print the corresponding label: 1 = Start, 2 = Settings, 3 = Exit. If the option is not in the range, print "Invalid Option".
14. Planet Order: Given a number from 1 to 8, print the corresponding planet's name in order from the Sun: 1 = Mercury, 2 = Venus, ..., 8 = Neptune. If the number is outside 1–8, print "Invalid planet number".
15. File Extension: Given a file extension:
    • If it's ".jpg", ".png", or ".gif", print "Image File"
    • If it's ".mp4" or ".mkv", print "Video File"
    • If it's ".pdf", print "Document File"
    • Otherwise, print "Unknown File Type"

🧠 Switch-Case Challenge Solutions

public class SwitchCaseChallengerAnswers {
    public static void main(String[] args) {

        // Q1: Day of the Week
        int day = 3;
        switch (day) {
            case 1: System.out.println("Q1 - Monday"); break;
            case 2: System.out.println("Q1 - Tuesday"); break;
            case 3: System.out.println("Q1 - Wednesday"); break;
            case 4: System.out.println("Q1 - Thursday"); break;
            case 5: System.out.println("Q1 - Friday"); break;
            case 6: System.out.println("Q1 - Saturday"); break;
            case 7: System.out.println("Q1 - Sunday"); break;
            default: System.out.println("Q1 - Invalid day");
        }

        // Q2: Grade Evaluator
        char grade = 'B';
        switch (grade) {
            case 'A': System.out.println("Q2 - Excellent"); break;
            case 'B': System.out.println("Q2 - Good"); break;
            case 'C': System.out.println("Q2 - Average"); break;
            case 'D': System.out.println("Q2 - Below Average"); break;
            case 'F': System.out.println("Q2 - Fail"); break;
            default: System.out.println("Q2 - Invalid grade");
        }

        // Q3: Month Name
        int month = 8;
        switch (month) {
            case 1: System.out.println("Q3 - January"); break;
            case 2: System.out.println("Q3 - February"); break;
            case 3: System.out.println("Q3 - March"); break;
            case 4: System.out.println("Q3 - April"); break;
            case 5: System.out.println("Q3 - May"); break;
            case 6: System.out.println("Q3 - June"); break;
            case 7: System.out.println("Q3 - July"); break;
            case 8: System.out.println("Q3 - August"); break;
            case 9: System.out.println("Q3 - September"); break;
            case 10: System.out.println("Q3 - October"); break;
            case 11: System.out.println("Q3 - November"); break;
            case 12: System.out.println("Q3 - December"); break;
            default: System.out.println("Q3 - Invalid month");
        }

        // Q4: Traffic Signal
        String signal = "green";
        switch (signal) {
            case "red": System.out.println("Q4 - Stop"); break;
            case "yellow": System.out.println("Q4 - Wait"); break;
            case "green": System.out.println("Q4 - Go"); break;
            default: System.out.println("Q4 - Invalid signal");
        }

        // Q5: Calculator Operation
        int a = 12, b = 4;
        char operator = '/';
        switch (operator) {
            case '+': System.out.println("Q5 - " + (a + b)); break;
            case '-': System.out.println("Q5 - " + (a - b)); break;
            case '*': System.out.println("Q5 - " + (a * b)); break;
            case '/': System.out.println("Q5 - " + (b != 0 ? (a / b) : "Cannot divide by zero")); break;
            default: System.out.println("Q5 - Invalid operator");
        }

        // Q6: Browser Detection
        String browser = "Safari";
        switch (browser) {
            case "Chrome":
            case "Firefox":
            case "Safari":
                System.out.println("Q6 - Supported");
                break;
            default:
                System.out.println("Q6 - Not supported");
        }

        // Q7: Vowel or Consonant
        char ch = 'O';
        switch (Character.toLowerCase(ch)) {
            case 'a':
            case 'e':
            case 'i':
            case 'o':
            case 'u':
                System.out.println("Q7 - Vowel");
                break;
            default:
                System.out.println("Q7 - Consonant");
        }

        // Q8: Level Description
        int level = 2;
        switch (level) {
            case 1: System.out.println("Q8 - Easy"); break;
            case 2: System.out.println("Q8 - Medium"); break;
            case 3: System.out.println("Q8 - Hard"); break;
            default: System.out.println("Q8 - Unknown Level");
        }

        // Q9: Shape Sides
        String shape = "Square";
        switch (shape) {
            case "Triangle": System.out.println("Q9 - 3 sides"); break;
            case "Square": System.out.println("Q9 - 4 sides"); break;
            case "Pentagon": System.out.println("Q9 - 5 sides"); break;
            default: System.out.println("Q9 - Unknown shape");
        }

        // Q10: Currency Symbol
        String currency = "JPY";
        switch (currency) {
            case "USD": System.out.println("Q10 - $ (USD)"); break;
            case "EUR": System.out.println("Q10 - € (EUR)"); break;
            case "JPY": System.out.println("Q10 - ¥ (JPY)"); break;
            default: System.out.println("Q10 - Unknown currency");
        }

        // Q11: Season Finder
        String monthName = "Jan";
        switch (monthName) {
            case "Dec":
            case "Jan":
            case "Feb":
                System.out.println("Q11 - Winter"); break;
            case "Mar":
            case "Apr":
            case "May":
                System.out.println("Q11 - Spring"); break;
            case "Jun":
            case "Jul":
            case "Aug":
                System.out.println("Q11 - Summer"); break;
            case "Sep":
            case "Oct":
            case "Nov":
                System.out.println("Q11 - Autumn"); break;
            default:
                System.out.println("Q11 - Invalid month");
        }

        // Q12: Language Greeting
        String language = "Spanish";
        switch (language) {
            case "English": System.out.println("Q12 - Hello"); break;
            case "Spanish": System.out.println("Q12 - Hola"); break;
            case "French": System.out.println("Q12 - Bonjour"); break;
            default: System.out.println("Q12 - Language not supported");
        }

        // Q13: Menu Selection
        int option = 2;
        switch (option) {
            case 1: System.out.println("Q13 - Start"); break;
            case 2: System.out.println("Q13 - Settings"); break;
            case 3: System.out.println("Q13 - Exit"); break;
            default: System.out.println("Q13 - Invalid Option");
        }

        // Q14: Planet Order
        int planetNumber = 4;
        switch (planetNumber) {
            case 1: System.out.println("Q14 - Mercury"); break;
            case 2: System.out.println("Q14 - Venus"); break;
            case 3: System.out.println("Q14 - Earth"); break;
            case 4: System.out.println("Q14 - Mars"); break;
            case 5: System.out.println("Q14 - Jupiter"); break;
            case 6: System.out.println("Q14 - Saturn"); break;
            case 7: System.out.println("Q14 - Uranus"); break;
            case 8: System.out.println("Q14 - Neptune"); break;
            default: System.out.println("Q14 - Invalid planet number");
        }

        // Q15: File Type Detector
        String fileExt = ".pdf";
        switch (fileExt) {
            case ".jpg":
            case ".png":
            case ".gif":
                System.out.println("Q15 - Image File"); break;
            case ".mp4":
            case ".mkv":
                System.out.println("Q15 - Video File"); break;
            case ".pdf":
                System.out.println("Q15 - Document File"); break;
            default:
                System.out.println("Q15 - Unknown File Type");
        }
    }
}

import java.util.Scanner;

public class SwitchCaseChallengerUserInput {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        // Q1: Day of the Week
        System.out.print("Q1 - Enter a number (1-7) for day of the week: ");
        int day = sc.nextInt();
        switch (day) {
            case 1: System.out.println("Q1 - Monday"); break;
            case 2: System.out.println("Q1 - Tuesday"); break;
            case 3: System.out.println("Q1 - Wednesday"); break;
            case 4: System.out.println("Q1 - Thursday"); break;
            case 5: System.out.println("Q1 - Friday"); break;
            case 6: System.out.println("Q1 - Saturday"); break;
            case 7: System.out.println("Q1 - Sunday"); break;
            default: System.out.println("Q1 - Invalid day");
        }

        // Q2: Grade Evaluator
        System.out.print("Q2 - Enter a grade (A/B/C/D/F): ");
        char grade = sc.next().toUpperCase().charAt(0);
        switch (grade) {
            case 'A': System.out.println("Q2 - Excellent"); break;
            case 'B': System.out.println("Q2 - Good"); break;
            case 'C': System.out.println("Q2 - Average"); break;
            case 'D': System.out.println("Q2 - Below Average"); break;
            case 'F': System.out.println("Q2 - Fail"); break;
            default: System.out.println("Q2 - Invalid grade");
        }

        // Q3: Month Name
        System.out.print("Q3 - Enter a number (1-12) for month: ");
        int month = sc.nextInt();
        switch (month) {
            case 1: System.out.println("Q3 - January"); break;
            case 2: System.out.println("Q3 - February"); break;
            case 3: System.out.println("Q3 - March"); break;
            case 4: System.out.println("Q3 - April"); break;
            case 5: System.out.println("Q3 - May"); break;
            case 6: System.out.println("Q3 - June"); break;
            case 7: System.out.println("Q3 - July"); break;
            case 8: System.out.println("Q3 - August"); break;
            case 9: System.out.println("Q3 - September"); break;
            case 10: System.out.println("Q3 - October"); break;
            case 11: System.out.println("Q3 - November"); break;
            case 12: System.out.println("Q3 - December"); break;
            default: System.out.println("Q3 - Invalid month");
        }

        // Q4: Traffic Signal
        System.out.print("Q4 - Enter traffic signal color (red/yellow/green): ");
        String signal = sc.next().toLowerCase();
        switch (signal) {
            case "red": System.out.println("Q4 - Stop"); break;
            case "yellow": System.out.println("Q4 - Wait"); break;
            case "green": System.out.println("Q4 - Go"); break;
            default: System.out.println("Q4 - Invalid signal");
        }

        // Q5: Calculator Operation
        System.out.print("Q5 - Enter two integers: ");
        int a = sc.nextInt();
        int b = sc.nextInt();
        System.out.print("Q5 - Enter an operator (+, -, *, /): ");
        char operator = sc.next().charAt(0);
        switch (operator) {
            case '+': System.out.println("Q5 - " + (a + b)); break;
            case '-': System.out.println("Q5 - " + (a - b)); break;
            case '*': System.out.println("Q5 - " + (a * b)); break;
            case '/':
                if (b != 0) System.out.println("Q5 - " + (a / b));
                else System.out.println("Q5 - Cannot divide by zero");
                break;
            default: System.out.println("Q5 - Invalid operator");
        }

        // Q6: Browser Detection
        System.out.print("Q6 - Enter a browser name (Chrome/Firefox/Safari): ");
        String browser = sc.next();
        switch (browser) {
            case "Chrome":
            case "Firefox":
            case "Safari":
                System.out.println("Q6 - Supported"); break;
            default:
                System.out.println("Q6 - Not supported");
        }

        // Q7: Vowel or Consonant
        System.out.print("Q7 - Enter an alphabet character: ");
        char ch = sc.next().toLowerCase().charAt(0);
        switch (ch) {
            case 'a': case 'e': case 'i': case 'o': case 'u':
                System.out.println("Q7 - Vowel"); break;
            default:
                System.out.println("Q7 - Consonant");
        }

        // Q8: Level Description
        System.out.print("Q8 - Enter a level (1-3): ");
        int level = sc.nextInt();
        switch (level) {
            case 1: System.out.println("Q8 - Easy"); break;
            case 2: System.out.println("Q8 - Medium"); break;
            case 3: System.out.println("Q8 - Hard"); break;
            default: System.out.println("Q8 - Unknown Level");
        }

        // Q9: Shape Sides
        System.out.print("Q9 - Enter a shape (Triangle/Square/Pentagon): ");
        String shape = sc.next();
        switch (shape) {
            case "Triangle": System.out.println("Q9 - 3 sides"); break;
            case "Square": System.out.println("Q9 - 4 sides"); break;
            case "Pentagon": System.out.println("Q9 - 5 sides"); break;
            default: System.out.println("Q9 - Unknown shape");
        }

        // Q10: Currency Symbol
        System.out.print("Q10 - Enter a currency code (USD/EUR/JPY): ");
        String currency = sc.next();
        switch (currency) {
            case "USD": System.out.println("Q10 - $ (USD)"); break;
            case "EUR": System.out.println("Q10 - € (EUR)"); break;
            case "JPY": System.out.println("Q10 - ¥ (JPY)"); break;
            default: System.out.println("Q10 - Unknown currency");
        }

        // Q11: Season Finder
        System.out.print("Q11 - Enter month short name (Jan-Dec): ");
        String monthName = sc.next();
        switch (monthName) {
            case "Dec": case "Jan": case "Feb":
                System.out.println("Q11 - Winter"); break;
            case "Mar": case "Apr": case "May":
                System.out.println("Q11 - Spring"); break;
            case "Jun": case "Jul": case "Aug":
                System.out.println("Q11 - Summer"); break;
            case "Sep": case "Oct": case "Nov":
                System.out.println("Q11 - Autumn"); break;
            default:
                System.out.println("Q11 - Invalid month");
        }

        // Q12: Language Greeting
        System.out.print("Q12 - Enter a language (English/Spanish/French): ");
        String language = sc.next();
        switch (language) {
            case "English": System.out.println("Q12 - Hello"); break;
            case "Spanish": System.out.println("Q12 - Hola"); break;
            case "French": System.out.println("Q12 - Bonjour"); break;
            default: System.out.println("Q12 - Language not supported");
        }

        // Q13: Menu Selection
        System.out.print("Q13 - Enter menu option (1-3): ");
        int option = sc.nextInt();
        switch (option) {
            case 1: System.out.println("Q13 - Start"); break;
            case 2: System.out.println("Q13 - Settings"); break;
            case 3: System.out.println("Q13 - Exit"); break;
            default: System.out.println("Q13 - Invalid Option");
        }

        // Q14: Planet Order
        System.out.print("Q14 - Enter planet number (1-8): ");
        int planetNumber = sc.nextInt();
        switch (planetNumber) {
            case 1: System.out.println("Q14 - Mercury"); break;
            case 2: System.out.println("Q14 - Venus"); break;
            case 3: System.out.println("Q14 - Earth"); break;
            case 4: System.out.println("Q14 - Mars"); break;
            case 5: System.out.println("Q14 - Jupiter"); break;
            case 6: System.out.println("Q14 - Saturn"); break;
            case 7: System.out.println("Q14 - Uranus"); break;
            case 8: System.out.println("Q14 - Neptune"); break;
            default: System.out.println("Q14 - Invalid planet number");
        }

        // Q15: File Type Detector
        System.out.print("Q15 - Enter a file extension (e.g., .jpg, .pdf): ");
        String fileExt = sc.next();
        switch (fileExt) {
            case ".jpg": case ".png": case ".gif":
                System.out.println("Q15 - Image File"); break;
            case ".mp4": case ".mkv":
                System.out.println("Q15 - Video File"); break;
            case ".pdf":
                System.out.println("Q15 - Document File"); break;
            default:
                System.out.println("Q15 - Unknown File Type");
        }

        sc.close();
    }
}

🔁 15 Best for Loop Concept Challenges

1. Print Numbers from 1 to 10: Write a program using a for loop that prints all numbers starting from 1 up to 10, each on a new line.
2. Sum of First N Natural Numbers: Ask the user to enter a number N. Use a loop to calculate and print the sum of numbers from 1 to N.
3. Print Even Numbers from 1 to 50: Use a for loop to print all even numbers between 1 and 50 (inclusive), separated by spaces.
4. Multiplication Table Generator: Prompt the user to enter a number, then print its multiplication table from 1 to 10 using a for loop.
5. Factorial Calculator: Ask the user to enter a number N. Use a loop to compute the factorial of N (i.e., N!) and display the result.
6. Print Numbers in Reverse: Write a loop that prints numbers from 10 down to 1 in descending order, all on one line separated by spaces.
7. Square of Numbers from 1 to 10: Use a loop to print each number from 1 to 10 along with its square in the format: number^2 = result.
8. Count Multiples of 3 Between 1 and 100: Use a loop to count how many numbers from 1 to 100 are divisible by 3 and print that count.
9. Sum of Odd Numbers up to N: Ask the user to enter a number N, then use a loop to calculate and print the sum of all odd numbers from 1 to N.
10. Pattern Printing - Star Triangle: Use nested for loops to print a right-angled triangle of asterisks (*) with 5 rows.
11. Pattern Printing - Number Triangle: Use nested loops to print a triangle with numbers starting from 1 and increasing consecutively across 5 rows.
12. Skip Multiples of 4: Use a loop to print numbers from 1 to 20, but skip (do not print) numbers that are divisible by 4.
13. Stop Loop on Specific Match: Print numbers from 1 to 100 using a loop, but stop the loop (use break) when the number 42 is reached.
14. Sum of Digits of a Number: Ask the user to input a number. Use a loop to compute and print the sum of its individual digits.
15. Fibonacci Series Generator: Prompt the user to enter how many terms of the Fibonacci sequence to print. Then use a loop to display the first N terms of the sequence.

🧮 Java for Loop Challenge Solutions

public class ForLoopChallengerAnswers {
    public static void main(String[] args) {

        // 1. Print 1 to 10
        for (int i = 1; i <= 10; i++) {
            System.out.println("Q1 - " + i);
        }

        // 2. Sum of First N Numbers
        int n = 10, sum = 0;
        for (int i = 1; i <= n; i++) {
            sum += i;
        }
        System.out.println("Q2 - Sum: " + sum);

        // 3. Even Numbers from 1 to 50
        System.out.print("Q3 - Even Numbers: ");
        for (int i = 2; i <= 50; i += 2) {
            System.out.print(i + " ");
        }
        System.out.println();

        // 4. Multiplication Table
        int num = 7;
        for (int i = 1; i <= 10; i++) {
            System.out.println("Q4 - " + num + " x " + i + " = " + (num * i));
        }

        // 5. Factorial Calculator
        int fact = 1, x = 5;
        for (int i = 1; i <= x; i++) {
            fact *= i;
        }
        System.out.println("Q5 - Factorial: " + fact);

        // 6. Reverse Print
        System.out.print("Q6 - Reverse: ");
        for (int i = 10; i >= 1; i--) {
            System.out.print(i + " ");
        }
        System.out.println();

        // 7. Square of Numbers 1-10
        for (int i = 1; i <= 10; i++) {
            System.out.println("Q7 - " + i + "^2 = " + (i * i));
        }

        // 8. Count Multiples of 3 (1 to 100)
        int count = 0;
        for (int i = 1; i <= 100; i++) {
            if (i % 3 == 0) count++;
        }
        System.out.println("Q8 - Multiples of 3: " + count);

        // 9. Sum of Odd Numbers up to N
        int oddSum = 0;
        for (int i = 1; i <= n; i += 2) {
            oddSum += i;
        }
        System.out.println("Q9 - Sum of odd numbers: " + oddSum);

        // 10. Pattern: Stars Triangle (5 rows)
        System.out.println("Q10 - Star Triangle:");
        for (int i = 1; i <= 5; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print("* ");
            }
            System.out.println();
        }

        // 11. Pattern: Numbers Triangle (5 rows)
        System.out.println("Q11 - Number Triangle:");
        int number = 1;
        for (int i = 1; i <= 5; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print(number++ + " ");
            }
            System.out.println();
        }

        // 12. Skip Multiples of 4
        System.out.print("Q12 - Skipping multiples of 4: ");
        for (int i = 1; i <= 20; i++) {
            if (i % 4 == 0) continue;
            System.out.print(i + " ");
        }
        System.out.println();

        // 13. Break on Match
        System.out.print("Q13 - Break at 42: ");
        for (int i = 1; i <= 100; i++) {
            if (i == 42) break;
            System.out.print(i + " ");
        }
        System.out.println();

        // 14. Sum of Digits
        int numToSum = 1234;
        int digitSum = 0;
        for (; numToSum > 0; numToSum /= 10) {
            digitSum += numToSum % 10;
        }
        System.out.println("Q14 - Digit Sum: " + digitSum);

        // 15. Fibonacci Series (first N terms)
        System.out.print("Q15 - Fibonacci Series: ");
        int terms = 10, a = 0, b = 1;
        for (int i = 1; i <= terms; i++) {
            System.out.print(a + " ");
            int next = a + b;
            a = b;
            b = next;
        }
        System.out.println();
    }
}

import java.util.Scanner;

public class ForLoopChallengerUserInput {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        // 1. Print 1 to 10
        System.out.println("Q1 - Numbers from 1 to 10:");
        for (int i = 1; i <= 10; i++) {
            System.out.println(i);
        }

        // 2. Sum of First N Numbers
        System.out.print("Q2 - Enter N for sum from 1 to N: ");
        int n = sc.nextInt();
        int sum = 0;
        for (int i = 1; i <= n; i++) {
            sum += i;
        }
        System.out.println("Sum = " + sum);

        // 3. Even Numbers from 1 to 50
        System.out.print("Q3 - Even Numbers from 1 to 50: ");
        for (int i = 2; i <= 50; i += 2) {
            System.out.print(i + " ");
        }
        System.out.println();

        // 4. Multiplication Table
        System.out.print("Q4 - Enter a number for multiplication table: ");
        int num = sc.nextInt();
        for (int i = 1; i <= 10; i++) {
            System.out.println(num + " x " + i + " = " + (num * i));
        }

        // 5. Factorial Calculator
        System.out.print("Q5 - Enter a number to calculate factorial: ");
        int x = sc.nextInt();
        int fact = 1;
        for (int i = 1; i <= x; i++) {
            fact *= i;
        }
        System.out.println("Factorial = " + fact);

        // 6. Reverse Print
        System.out.print("Q6 - Numbers from 10 to 1: ");
        for (int i = 10; i >= 1; i--) {
            System.out.print(i + " ");
        }
        System.out.println();

        // 7. Square of Numbers from 1 to 10
        System.out.println("Q7 - Squares of numbers 1 to 10:");
        for (int i = 1; i <= 10; i++) {
            System.out.println(i + "^2 = " + (i * i));
        }

        // 8. Count Multiples of 3 (1 to 100)
        int count = 0;
        for (int i = 1; i <= 100; i++) {
            if (i % 3 == 0) count++;
        }
        System.out.println("Q8 - Count of multiples of 3 from 1 to 100: " + count);

        // 9. Sum of Odd Numbers up to N
        System.out.print("Q9 - Enter N to find sum of odd numbers up to N: ");
        int oddN = sc.nextInt();
        int oddSum = 0;
        for (int i = 1; i <= oddN; i += 2) {
            oddSum += i;
        }
        System.out.println("Sum of odd numbers = " + oddSum);

        // 10. Pattern: Stars Triangle
        System.out.println("Q10 - Star Triangle (5 rows):");
        for (int i = 1; i <= 5; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print("* ");
            }
            System.out.println();
        }

        // 11. Pattern: Numbers Triangle
        System.out.println("Q11 - Number Triangle (5 rows):");
        int number = 1;
        for (int i = 1; i <= 5; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print(number++ + " ");
            }
            System.out.println();
        }

        // 12. Skip Multiples of 4
        System.out.print("Q12 - Numbers from 1 to 20 (skipping multiples of 4): ");
        for (int i = 1; i <= 20; i++) {
            if (i % 4 == 0) continue;
            System.out.print(i + " ");
        }
        System.out.println();

        // 13. Break on Match
        System.out.print("Q13 - Numbers from 1 to 100 (stop at 42): ");
        for (int i = 1; i <= 100; i++) {
            if (i == 42) break;
            System.out.print(i + " ");
        }
        System.out.println();

        // 14. Sum of Digits
        System.out.print("Q14 - Enter a number to sum its digits: ");
        int numToSum = sc.nextInt();
        int digitSum = 0;
        for (; numToSum > 0; numToSum /= 10) {
            digitSum += numToSum % 10;
        }
        System.out.println("Digit sum = " + digitSum);

        // 15. Fibonacci Series
        System.out.print("Q15 - Enter N to print first N Fibonacci numbers: ");
        int terms = sc.nextInt();
        int a = 0, b = 1;
        System.out.print("Fibonacci Series: ");
        for (int i = 1; i <= terms; i++) {
            System.out.print(a + " ");
            int next = a + b;
            a = b;
            b = next;
        }
        System.out.println();

        sc.close();
    }
}

🔁 15 Best while Loop Concept Challenges

1. Print Numbers 1 to 10: Write a program that uses a while loop to print numbers from 1 to 10.
2. Countdown from 10 to 1: Use a while loop to print a countdown from 10 down to 1.
3. Add Until Total Reaches 100: Keep asking the user to input numbers and keep adding them until the total is 100 or more.
4. Reverse the Digits of a Number: Input a number and reverse its digits using a while loop.
5. Count the Number of Digits: Write a program that counts how many digits are in a given integer using a while loop.
6. Check if a Number is a Palindrome: Check whether a given number reads the same backward as forward (e.g., 1221).
7. Sum the Digits of a Number: Calculate and print the sum of all digits in a number using a while loop.
8. Multiplication Table Until Exit: Continuously print the multiplication table for user input until the user enters -1 to exit.
9. Guess the Secret Number: Let the user guess a secret number until they guess it correctly.
10. Print Powers of Two up to 1024: Use a while loop to print powers of 2 starting from 1 up to 1024.
11. Generate Fibonacci Numbers Until a Limit: Print all Fibonacci numbers less than a given limit (e.g., 100) using a while loop.
12. Check if a Number is an Armstrong Number: Determine whether a 3-digit number is an Armstrong number (e.g., 153 → 1³ + 5³ + 3³ = 153).
13. Sum of Even Numbers from 1 to 100: Use a while loop to calculate the sum of all even numbers from 1 to 100.
14. Find All Factors of a Number: Print all positive integers that divide a given number exactly using a while loop.
15. User Login with 3 Attempts: Allow the user up to 3 attempts to enter the correct password. If incorrect after 3 tries, lock the account.

🧮 Java while Loop Challenge Solutions

import java.util.Scanner;

public class WhileLoopChallengerAnswers {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        // Q1: Print 1 to 10
        System.out.println("Q1 - Printing numbers from 1 to 10:");
        int i = 1;
        while (i <= 10) {
            System.out.println(i);
            i++;
        }

        // Q2: Countdown
        System.out.println("\nQ2 - Countdown from 10 to 1:");
        int n = 10;
        while (n >= 1) {
            System.out.println(n);
            n--;
        }

        // Q3: Sum Until 100
        System.out.println("\nQ3 - Keep entering numbers to reach or exceed a sum of 100:");
        int sum = 0;
        while (sum < 100) {
            System.out.print("Enter number: ");
            int input = sc.nextInt();
            sum += input;
        }
        System.out.println("Total sum: " + sum);

        // Q4: Reverse Digits
        System.out.print("\nQ4 - Enter a number to reverse its digits: ");
        int num = sc.nextInt();
        int rev = 0;
        while (num != 0) {
            int digit = num % 10;
            rev = rev * 10 + digit;
            num /= 10;
        }
        System.out.println("Reversed number: " + rev);

        // Q5: Count Digits
        System.out.print("\nQ5 - Enter a number to count its digits: ");
        int countNum = sc.nextInt();
        int count = 0;
        while (countNum != 0) {
            countNum /= 10;
            count++;
        }
        System.out.println("Number of digits: " + count);

        // Q6: Check Palindrome
        System.out.print("\nQ6 - Enter a number to check if it's a palindrome: ");
        int original = sc.nextInt();
        int temp = original;
        int reversed = 0;
        while (temp != 0) {
            int digit = temp % 10;
            reversed = reversed * 10 + digit;
            temp /= 10;
        }
        System.out.println(original == reversed ? "Palindrome" : "Not Palindrome");

        // Q7: Sum of Digits
        System.out.print("\nQ7 - Enter a number to find sum of its digits: ");
        int digitSumNum = sc.nextInt();
        int digitSum = 0;
        while (digitSumNum != 0) {
            digitSum += digitSumNum % 10;
            digitSumNum /= 10;
        }
        System.out.println("Sum of digits: " + digitSum);

        // Q8: Table Until Exit
        System.out.println("\nQ8 - Enter numbers to print their multiplication table (-1 to exit):");
        int tableInput;
        do {
            System.out.print("Enter number: ");
            tableInput = sc.nextInt();
            int t = 1;
            while (tableInput != -1 && t <= 10) {
                System.out.println(tableInput + " x " + t + " = " + (tableInput * t));
                t++;
            }
        } while (tableInput != -1);

        // Q9: Guess the Number
        System.out.println("\nQ9 - Guess the secret number between 1 and 10:");
        int secret = 7;
        int guess;
        do {
            System.out.print("Your guess: ");
            guess = sc.nextInt();
        } while (guess != secret);
        System.out.println("Correct!");

        // Q10: Power of Two
        System.out.println("\nQ10 - Powers of 2 up to 1024:");
        int power = 1;
        while (power <= 1024) {
            System.out.println(power);
            power *= 2;
        }

        // Q11: Fibonacci Until Limit
        System.out.print("\nQ11 - Enter a limit to generate Fibonacci numbers: ");
        int limit = sc.nextInt();
        int a = 0, b = 1;
        System.out.print("Fibonacci series: ");
        while (a < limit) {
            System.out.print(a + " ");
            int next = a + b;
            a = b;
            b = next;
        }
        System.out.println();

        // Q12: Check Armstrong Number
        System.out.print("\nQ12 - Enter a 3-digit number to check if it's an Armstrong number: ");
        int arm = sc.nextInt();
        int originalArm = arm;
        int result = 0;
        while (arm != 0) {
            int digit = arm % 10;
            result += digit * digit * digit;
            arm /= 10;
        }
        System.out.println(result == originalArm ? "Armstrong number" : "Not Armstrong");

        // Q13: Even Sum
        System.out.println("\nQ13 - Sum of even numbers from 1 to 100:");
        int even = 1;
        int evenSum = 0;
        while (even <= 100) {
            if (even % 2 == 0)
                evenSum += even;
            even++;
        }
        System.out.println("Sum of even numbers: " + evenSum);

        // Q14: Factor Finder
        System.out.print("\nQ14 - Enter a number to find its factors: ");
        int factNum = sc.nextInt();
        int f = 1;
        System.out.println("Factors of " + factNum + ":");
        while (f <= factNum) {
            if (factNum % f == 0)
                System.out.println(f);
            f++;
        }

        // Q15: User Login Attempts
        String correctPass = "admin123";
        int attempts = 0;
        System.out.println("\nQ15 - Login system (3 attempts):");
        while (attempts < 3) {
            System.out.print("Enter password: ");
            String input = sc.next();
            if (input.equals(correctPass)) {
                System.out.println("Access Granted");
                break;
            }
            attempts++;
        }
        if (attempts == 3) {
            System.out.println("Account Locked");
        }

        sc.close();
    }
}

🔁 15 Best do-while Concept Challenges

1. Print numbers from 1 to 10: Use a do-while loop to print numbers from 1 to 10. No user input is required.
2. Sum of first N numbers: Prompt the user to enter a number N. Use a do-while loop to find and print the sum of numbers from 1 to N.
3. Multiplication table: Ask the user to enter a number. Use a do-while loop to print its multiplication table up to 10.
4. Reverse a number: Prompt the user to enter a number. Use a do-while loop to reverse the digits of that number (e.g., input: 1234 → output: 4321).
5. Palindrome check: Ask the user to input a number. Use a do-while loop to check whether the number is a palindrome (reads the same forwards and backwards).
6. Digit count: Prompt the user to enter a number. Use a do-while loop to count how many digits the number has.
7. Even numbers from 1 to 50: Use a do-while loop to print all even numbers between 1 and 50 (inclusive). No user input required.
8. Factorial: Ask the user to enter a number. Use a do-while loop to calculate and print its factorial (e.g., 5 → 120).
9. Sum of digits: Prompt the user to enter a number. Use a do-while loop to compute and display the sum of its digits.
10. Positive input prompt: Use a do-while loop to repeatedly ask the user to enter a number until they provide a positive number.
11. Decimal to binary conversion: Ask the user to enter a decimal number. Use a do-while loop to convert and display its binary representation (as an integer).
12. Star pattern: Prompt the user to enter the number of rows. Use nested do-while loops to print a right-angled triangle pattern of stars (*) up to that number of rows.
13. Countdown: Use a do-while loop to print a countdown from 10 to 1. No user input is required.
14. Fibonacci series: Ask the user how many terms of the Fibonacci series to print. Use a do-while loop to generate and display that many terms.
15. Sum of odd numbers: Prompt the user to enter a number N. Use a do-while loop to calculate and print the sum of all odd numbers from 1 to N.

🧮 Java do-while Challenge Solutions

import java.util.Scanner;

public class DoWhileChallengerUserInput {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        // Q1: Print 1 to 10
        System.out.println("Q1 - Printing numbers from 1 to 10:");
        int i = 1;
        do {
            System.out.print(i + " ");
            i++;
        } while (i <= 10);
        System.out.println("\n");

        // Q2: Sum of first N numbers
        System.out.print("Q2 - Enter N to find sum from 1 to N: ");
        int n = sc.nextInt();
        int sum = 0, j = 1;
        do {
            sum += j;
            j++;
        } while (j <= n);
        System.out.println("Sum: " + sum + "\n");

        // Q3: Multiplication table
        System.out.print("Q3 - Enter a number for its multiplication table: ");
        int num = sc.nextInt(), k = 1;
        do {
            System.out.println(num + " x " + k + " = " + (num * k));
            k++;
        } while (k <= 10);
        System.out.println();

        // Q4: Reverse a number
        System.out.print("Q4 - Enter a number to reverse: ");
        int number = sc.nextInt(), reversed = 0, temp = number;
        do {
            reversed = reversed * 10 + temp % 10;
            temp /= 10;
        } while (temp != 0);
        System.out.println("Reversed: " + reversed + "\n");

        // Q5: Palindrome check
        System.out.print("Q5 - Enter a number to check palindrome: ");
        int original = sc.nextInt(), rev = 0, copy = original;
        do {
            rev = rev * 10 + copy % 10;
            copy /= 10;
        } while (copy != 0);
        System.out.println(original == rev ? "Palindrome\n" : "Not a palindrome\n");

        // Q6: Digit count
        System.out.print("Q6 - Enter a number to count digits: ");
        int digitNum = sc.nextInt(), count = 0;
        do {
            count++;
            digitNum /= 10;
        } while (digitNum != 0);
        System.out.println("Digit count: " + count + "\n");

        // Q7: Even numbers from 1 to 50
        System.out.println("Q7 - Even numbers from 1 to 50:");
        int ev = 2;
        do {
            System.out.print(ev + " ");
            ev += 2;
        } while (ev <= 50);
        System.out.println("\n");

        // Q8: Factorial
        System.out.print("Q8 - Enter a number to find factorial: ");
        int x = sc.nextInt(), fact = 1, f = 1;
        do {
            fact *= f;
            f++;
        } while (f <= x);
        System.out.println("Factorial: " + fact + "\n");

        // Q9: Sum of digits
        System.out.print("Q9 - Enter a number to sum its digits: ");
        int numToSum = sc.nextInt(), digitSum = 0;
        do {
            digitSum += numToSum % 10;
            numToSum /= 10;
        } while (numToSum != 0);
        System.out.println("Digit sum: " + digitSum + "\n");

        // Q10: Positive input prompt simulation
        int input;
        do {
            System.out.print("Q10 - Enter a positive number: ");
            input = sc.nextInt();
        } while (input <= 0);
        System.out.println("Positive input accepted: " + input + "\n");

        // Q11: Decimal (Integer) to Binary
        System.out.print("Q11 - Enter a whole number (integer): ");
        if (sc.hasNextInt()) {
            int dec = sc.nextInt();
            int binary = 0, place = 1;
            int originalDec = dec;
        
            if (dec == 0) {
                binary = 0;
            } else {
                do {
                    int rem = dec % 2;
                    binary += rem * place;
                    place *= 10;
                    dec /= 2;
                } while (dec != 0);
            }
        
            System.out.println("Binary of " + originalDec + " is: " + binary + "\n");
        } else {
            System.out.println("Invalid input! Please enter a whole number (no decimal part).\n");
            sc.next(); // clear the invalid input
        }

        // Q11: Decimal (Floating-Point) to Binary
        System.out.print("Q11 - Enter a decimal number (e.g., 5.75): ");
        if (sc.hasNextDouble()) {
            double decimalInput = sc.nextDouble();
            int intPart = (int) decimalInput;
            double fracPart = decimalInput - intPart;
        
            // Convert integer part using do-while
            String intBinary = "";
            if (intPart == 0) {
                intBinary = "0";
            } else {
                do {
                    intBinary = (intPart % 2) + intBinary;
                    intPart /= 2;
                } while (intPart > 0);
            }
        
            // Convert fractional part using do-while
            String fracBinary = "";
            int limit = 10; // max digits after point
            int fracCount = 0;
        
            if (fracPart > 0) {
                do {
                    fracPart *= 2;
                    if (fracPart >= 1) {
                        fracBinary += "1";
                        fracPart -= 1;
                    } else {
                        fracBinary += "0";
                    }
                    fracCount++;
                } while (fracPart > 0 && fracCount < limit);
            }
        
            System.out.println("Binary of " + decimalInput + " is: " + intBinary +
                    (fracBinary.isEmpty() ? "" : "." + fracBinary) + "\n");
        
        } else {
            System.out.println("Invalid input! Please enter a numeric value.\n");
            sc.next(); // clear invalid input
        }

        // Q12: Star pattern
        System.out.print("Q12 - Enter number of rows for star pattern: ");
        int rows = sc.nextInt(), row = 1;
        do {
            int star = 1;
            do {
                System.out.print("* ");
                star++;
            } while (star <= row);
            System.out.println();
            row++;
        } while (row <= rows);
        System.out.println();

        // Q13: Countdown
        System.out.println("Q13 - Countdown from 10:");
        int down = 10;
        do {
            System.out.print(down + " ");
            down--;
        } while (down >= 1);
        System.out.println("\n");

        // Q14: Fibonacci series
        System.out.print("Q14 - Enter number of Fibonacci terms: ");
        int terms = sc.nextInt(), a = 0, b = 1, fib = 1;
        System.out.print("Fibonacci series: ");
        do {
            System.out.print(a + " ");
            int next = a + b;
            a = b;
            b = next;
            fib++;
        } while (fib <= terms);
        System.out.println("\n");

        // Q15: Sum of odd numbers
        System.out.print("Q15 - Enter a number to sum all odd numbers up to it: ");
        int limit = sc.nextInt(), odd = 1, oddSum = 0;
        do {
            oddSum += odd;
            odd += 2;
        } while (odd <= limit);
        System.out.println("Sum of odd numbers: " + oddSum);

        sc.close();
    }
}

🔢 Decimal to Binary Converter

🧠 Logic (Simple Explanation)

• We ask the user for a number.
• If it's a whole number (like 5 or 10), we use math to turn it into binary.
• If it's a number with a decimal (like 5.75), we split it into two parts and convert both separately.

🔁 Integer to Binary using do-while loop

// 🟢 Q11: Decimal (Integer) to Binary using do-while
System.out.print("Q11 - Enter a whole number (integer): "); // Ask the user for an integer input
if (sc.hasNextInt()) { // Check if the input is a valid whole number (integer)
    int dec = sc.nextInt(); // Read and store the input number
    int binary = 0, place = 1; // Initialize binary result and place value (1s, 10s, 100s...)
    int originalDec = dec; // Save original number for display later

    if (dec == 0) { // Special case: if the number is 0, binary is also 0
        binary = 0;
    } else {
        do { // Start do-while loop for conversion (runs at least once)
            int rem = dec % 2; // Get remainder when dividing by 2 (either 0 or 1)
            binary += rem * place; // Add the bit in the correct place value (units, tens, etc.)
            place *= 10; // Move to the next binary digit's place (like shifting left)
            dec /= 2; // Divide number by 2 for next iteration (integer division)
        } while (dec != 0); // Repeat until the number becomes 0
    }

    // Print the original number and its binary form
    System.out.println("Binary of " + originalDec + " is: " + binary + "\n");
} else {
    // If user entered an invalid input (like a string or decimal), show message
    System.out.println("Invalid input! Please enter a whole number (no decimal part).\n");
    sc.next(); // Clear the invalid input from the scanner buffer
}

📘 Easy Explanation:

• We keep dividing the number by 2 using a do-while loop.
• Each time, we get a remainder — either 0 or 1.
• We build the binary number by combining these remainders from right to left.
• place helps put binary digits in the right place: units, tens, hundreds.

🔁 Floating-Point (Decimal) to Binary using do-while loop

// 🟢 Q11: Decimal (Floating-Point) to Binary using do-while
System.out.print("Q11 - Enter a decimal number (e.g., 5.75): "); // Prompt user for decimal input
if (sc.hasNextDouble()) { // Check if the input is a valid decimal (integers are valid too)
    double decimalInput = sc.nextDouble(); // Read the input number
    int intPart = (int) decimalInput; // Extract the integer part
    double fracPart = decimalInput - intPart; // Extract the fractional part

    // --- Convert integer part using do-while ---
    String intBinary = ""; // To store binary of the integer part
    if (intPart == 0) {
        intBinary = "0"; // If 0, set binary as "0"
    } else {
        do {
            intBinary = (intPart % 2) + intBinary; // Add remainder to binary string
            intPart /= 2; // Move to next bit
        } while (intPart > 0); // Repeat until integer part becomes 0
    }

    // --- Convert fractional part using do-while ---
    String fracBinary = ""; // To store binary of fractional part
    int limit = 10; // Limit to max 10 fractional digits
    int fracCount = 0; // Counter for fractional digits

    if (fracPart > 0) {
        do {
            fracPart *= 2; // Multiply fraction by 2
            if (fracPart >= 1) {
                fracBinary += "1"; // Add 1 and subtract
                fracPart -= 1;
            } else {
                fracBinary += "0"; // Add 0
            }
            fracCount++; // Increase counter
        } while (fracPart > 0 && fracCount < limit); // Stop if fraction becomes 0 or limit reached
    }

    // Display final binary representation
    System.out.println("Binary of " + decimalInput + " is: " + intBinary +
            (fracBinary.isEmpty() ? "" : "." + fracBinary) + "\n");

} else {
    // If invalid input entered
    System.out.println("Invalid input! Please enter a numeric value.\n");
    sc.next(); // Clear invalid token
}

📘 Easy Explanation:

• We break the number into two parts: before and after the decimal point.
• For the integer part, we use do-while and divide repeatedly by 2 (same as above).
• For the fractional part:
  • Multiply by 2.
  • If the result is 1 or more, record a 1 and subtract 1.
  • If it's less than 1, record a 0.
  • Repeat (using do-while) up to 10 times or until the fraction becomes 0.
• The final binary is a combination of the two parts with a . in the middle.

✅ Sample Output:

Integer Input: 13
Binary: 1101

Decimal Input: 5.75
Integer part: 5 → 101
Fraction part: .75 → .11
Binary: 101.11

🔁 Integer to Binary using while loop

// Q11: Decimal (Integer) to Binary
System.out.print("Q11 - Enter a whole number (integer): ");  // Ask user to type a whole number (no decimals)

if (sc.hasNextInt()) {  // Check if the user typed a valid whole number
    int dec = sc.nextInt();  // Store the number in 'dec'
    int binary = 0, place = 1;  // 'binary' will hold our answer, 'place' tracks the position (1s, 10s, 100s)
    int originalDec = dec;  // Save the original number to show in the end

    while (dec != 0) {  // Keep going until the number becomes 0
        int rem = dec % 2;  // Divide by 2, get the remainder (this is one binary digit)
        binary += rem * place;  // Put the digit in the right position (units, tens, etc.)
        place *= 10;  // Move to the next place (e.g., 1 → 10 → 100)
        dec /= 2;  // Cut the number in half (integer division)
    }

    // Show the original and the result
    System.out.println("Binary of " + originalDec + " is: " + binary + "\n");
} else {
    // If they typed something wrong (like 5.2 or letters)
    System.out.println("Invalid input! Please enter a whole number (no decimal part).\n");
    sc.next(); // Clear the bad input
}

📘 Easy Explanation:

• We keep dividing the number by 2.
• Each time, we write down the remainder (0 or 1).
• We build the binary number by putting these remainders in reverse order.
• We use place to multiply the bits into correct position: 1s, 10s, 100s, etc.
• This is for **integers only**, not decimals like 5.1 or 7.75.

🔁 Floating-Point (Decimal) to Binary

// Q11: Decimal (Floating-Point) to Binary
System.out.print("Q11 - Enter a decimal number (e.g., 5.75): ");  // Ask user to enter a decimal number
if (sc.hasNextDouble()) {  // Check if they entered a valid decimal
    double decimalInput = sc.nextDouble();  // Read it
    int intPart = (int) decimalInput;  // Get the whole number part (e.g., from 5.75 → 5)
    double fracPart = decimalInput - intPart;  // Get the decimal part (e.g., from 5.75 → 0.75)

    // --- Convert the integer part to binary ---
    String intBinary = "";  // Store the binary result as text
    if (intPart == 0) {
        intBinary = "0";  // If the whole part is 0, just write 0
    } else {
        while (intPart > 0) {
            intBinary = (intPart % 2) + intBinary;  // Add remainder in front each time
            intPart /= 2;
        }
    }

    // --- Convert the fractional part to binary ---
    String fracBinary = "";
    int limit = 10; // Only go 10 digits after decimal point to avoid infinite loops

    while (fracPart > 0 && fracBinary.length() < limit) {
        fracPart *= 2;  // Multiply the fraction by 2
        if (fracPart >= 1) {
            fracBinary += "1";  // If it's 1 or more, add a 1
            fracPart -= 1;      // Remove the 1
        } else {
            fracBinary += "0";  // If less than 1, just add 0
        }
    }

    // Join the two parts together and print
    System.out.println("Binary of " + decimalInput + " is: " + intBinary + 
        (fracBinary.isEmpty() ? "" : "." + fracBinary) + "\n");
} else {
    System.out.println("Invalid input! Please enter a numeric value.\n");
    sc.next(); // clear invalid input
}

📘 Easy Explanation:

• We split the number into two parts: before and after the dot.
• Example: 5.75 → intPart = 5, fracPart = 0.75
• Convert the whole number (5) like before (5 → 101)
• To convert the decimal part (0.75):
  • Multiply it by 2 → 0.75 * 2 = 1.5 → record 1
  • Take the .5 → 0.5 * 2 = 1.0 → record 1
  • Done! → 0.75 → 11 in binary
• Final answer: 5.75 → 101.11

✅ Sample Output:

Integer Input: 13
Binary: 1101

Decimal Input: 5.75
Integer part: 5 → 101
Fraction part: .75 → .11
Binary: 101.11

🧪 Summary of Key Concepts:

Part	What It Does
% 2	Gets binary digit (remainder) — either 0 or 1
/ 2	Divides the number by 2 to move to the next binary digit (integer part)
* 2	Used to convert fractional part — shift left in binary terms
do-while	Loop runs at least once even if the condition is false initially — great for guaranteed input processing
while	Loop checks the condition first — safer when input may already be invalid
place	Tracks binary digit positions (1s, 10s, 100s...) — only used for integer-to-binary when storing result as a number
String	Used to build binary values with leading zeros or decimal points (especially for floating-point numbers)

🤔 Which is better: do-while or while?

• Use do-while when you want the loop to run at least once no matter what. Example: converting a number that could be zero.
• Use while when the condition must be true before running even the first time. It's slightly safer in input-sensitive scenarios.
• In binary conversion: both can work, but do-while is a better fit when converting 0 or small numbers (guarantees 1 run).

🧮 Factorial Calculator Using All Loop Types in Java

The factorial of a number n is the product of all positive integers from 1 to n.

🧠 Step-by-Step Explanation

• Input a number (e.g., 5).
• Initialize a variable fact = 1.
• Loop from 1 to that number and multiply fact by each value.
• Print the final value of fact after the loop ends.

🔁 Using for Loop

import java.util.Scanner;

public class FactorialFor {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter a number: ");
    int num = sc.nextInt();
    int fact = 1;

    for (int i = 1; i <= num; i++) {
      fact *= i;
    }

    System.out.println("Factorial of " + num + " is: " + fact);
  }
}

🔁 Using while Loop

import java.util.Scanner;

public class FactorialWhile {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter a number: ");
    int num = sc.nextInt();
    int fact = 1, i = 1;

    while (i <= num) {
      fact *= i;
      i++;
    }

    System.out.println("Factorial of " + num + " is: " + fact);
  }
}

🔁 Using do-while Loop

import java.util.Scanner;

public class FactorialDoWhile {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter a number: ");
    int num = sc.nextInt();
    int fact = 1, i = 1;

    do {
      fact *= i;
      i++;
    } while (i <= num);

    System.out.println("Factorial of " + num + " is: " + fact);
  }
}

✅ Summary

• Factorial is computed by multiplying a series of integers from 1 to n.
• Use any loop type depending on control and preference.
• for is best for fixed iterations; while and do-while are useful for variable conditions.

🔢 Fibonacci Series in Java

This section demonstrates how to print the Fibonacci Series using for, while, and do-while loops in Java. It also includes detailed step-by-step explanations inside the code as comments.

✅ 1. Using for loop

public class FibonacciFor {
  public static void main(String[] args) {
    int n = 10; // Total number of terms
    int a = 0, b = 1;

    System.out.print("Fibonacci Series using for loop: ");

    // Step 1: Print first two terms
    System.out.print(a + " " + b + " ");

    // Step 2: Loop from 2 to n-1
    for (int i = 2; i < n; i++) {
      int c = a + b;     // Next term is sum of previous two
      System.out.print(c + " ");
      a = b;             // Move b to a
      b = c;             // Move c to b
    }
  }
}

✅ 2. Using while loop

public class FibonacciWhile {
  public static void main(String[] args) {
    int n = 10;
    int a = 0, b = 1;
    int i = 2;

    System.out.print("Fibonacci Series using while loop: ");

    // Step 1: Print the first two terms
    System.out.print(a + " " + b + " ");

    // Step 2: Continue loop while i < n
    while (i < n) {
      int c = a + b;
      System.out.print(c + " ");
      a = b;
      b = c;
      i++; // Increment counter
    }
  }
}

✅ 3. Using do-while loop

public class FibonacciDoWhile {
  public static void main(String[] args) {
    int n = 10;
    int a = 0, b = 1;
    int i = 2;

    System.out.print("Fibonacci Series using do-while loop: ");

    // Step 1: Print the first two terms
    System.out.print(a + " " + b + " ");

    // Step 2: Use do-while to continue until n terms
    do {
      int c = a + b;
      System.out.print(c + " ");
      a = b;
      b = c;
      i++;
    } while (i < n);
  }
}

📌 Step-by-Step Logic

• Start with two variables: a = 0, b = 1
• Print the first two terms: 0 1
• Loop from 2 to n:
  • Calculate c = a + b
  • Print c
  • Update a = b, b = c
• Repeat until all n terms are printed

📈 Fibonacci Series Until a Given Limit

The Fibonacci sequence is a series of numbers where each number is the sum of the two preceding ones. The sequence starts from 0 and 1. Unlike the version with a fixed number of terms, here we stop when the next number would exceed a user-defined limit.

🔁 Using for Loop

// Step-by-step for-loop Fibonacci until a limit
int limit = 100;
int a = 0, b = 1;
System.out.print("Fibonacci series up to " + limit + ": ");
System.out.print(a + " " + b + " ");

for (int next = a + b; next <= limit; next = a + b) {
  System.out.print(next + " ");
  a = b;
  b = next;
}

• Start with two base numbers: 0 and 1.
• Print both starting numbers.
• Loop to calculate the next Fibonacci number until it exceeds the given limit.
• Update a and b accordingly in each iteration.

🔄 Using while Loop

// Step-by-step while-loop Fibonacci until a limit
int limit = 100;
int a = 0, b = 1;
System.out.print("Fibonacci series up to " + limit + ": ");
System.out.print(a + " " + b + " ");
int next = a + b;

while (next <= limit) {
  System.out.print(next + " ");
  a = b;
  b = next;
  next = a + b;
}

🔂 Using do-while Loop

// Step-by-step do-while-loop Fibonacci until a limit
int limit = 100;
int a = 0, b = 1;
System.out.print("Fibonacci series up to " + limit + ": ");
System.out.print(a + " " + b + " ");
int next = a + b;

do {
  System.out.print(next + " ");
  a = b;
  b = next;
  next = a + b;
} while (next <= limit);

This version ensures that at least one iteration happens even if the limit is small. Ideal when initial values must be printed regardless of conditions.

🧮 Factor Finder: Using All Loops

🔹 What Are Factors?

A factor of a number is any number that divides it exactly without leaving a remainder.

Example: Factors of 12 are 1, 2, 3, 4, 6, and 12.

🔁 Using for Loop

int num = 12;
System.out.println("Factors of " + num + " using for loop:");

for (int i = 1; i <= num; i++) {
    if (num % i == 0) {
        System.out.print(i + " ");
    }
}

✅ Explanation:

• Start loop from 1 to num
• Check if num % i == 0
• If yes, print i as a factor

🔁 Using while Loop

int num = 12;
int i = 1;
System.out.println("\nFactors of " + num + " using while loop:");

while (i <= num) {
    if (num % i == 0) {
        System.out.print(i + " ");
    }
    i++;
}

✅ Explanation:

• Initialize counter i = 1
• Repeat while i <= num
• Check divisibility and print factor
• Increment i

🔁 Using do-while Loop

int num = 12;
int i = 1;
System.out.println("\nFactors of " + num + " using do-while loop:");

do {
    if (num % i == 0) {
        System.out.print(i + " ");
    }
    i++;
} while (i <= num);

✅ Explanation:

• Start with i = 1
• Use do-while to ensure the loop runs at least once
• Check divisibility and print factor
• Increment i until i > num

✅ Output Example for num = 12:

Factors of 12 using for loop:
1 2 3 4 6 12

Factors of 12 using while loop:
1 2 3 4 6 12

Factors of 12 using do-while loop:
1 2 3 4 6 12

🔢 Digit Counter in Java

📝 Objective:

Count the number of digits in an integer using for, while, and do-while loops.

💡 Logic:

• Divide the number by 10 repeatedly until it becomes 0.
• Each division removes the last digit.
• Count how many times this happens.

📘 Example:

If the number is 4567, it has 4 digits.

✅ Using for Loop

int num = 4567;
int count = 0;

for (int temp = num; temp != 0; temp /= 10) {
    count++;
}

System.out.println("Digit Count: " + count);

🔍 Step-by-Step:

1. Initialize temp = num.
2. Loop: temp != 0.
3. On each iteration, remove the last digit using temp /= 10.
4. Increment count until temp becomes 0.

✅ Using while Loop

int num = 4567;
int count = 0;
int temp = num;

while (temp != 0) {
    count++;
    temp /= 10;
}

System.out.println("Digit Count: " + count);

🔍 Step-by-Step:

1. Copy num to temp.
2. Continue while temp != 0.
3. Each time: divide temp by 10 and increment count.

✅ Using do-while Loop

int num = 4567;
int count = 0;
int temp = num;

do {
    count++;
    temp /= 10;
} while (temp != 0);

System.out.println("Digit Count: " + count);

🔍 Step-by-Step:

1. This loop runs at least once, even if the number is 0.
2. Each loop: count++, then divide temp by 10.
3. Loop continues while temp != 0.

📌 Special Case:

• If num = 0, all loops must still return 1 digit.
• Fix: Add a condition to check if (num == 0), return 1 directly or handle it in do-while.

✅ Output:

Digit Count: 4

🔢 Digit Analysis

Below we calculate:

• Total number of digits
• Sum of digits
• Count of even and odd digits

🧮 Using for loop

int number = 4623;
int original = number;
int count = 0, even = 0, odd = 0, sum = 0;

for (; number > 0; number /= 10) {
  int digit = number % 10;
  count++;
  sum += digit;
  if (digit % 2 == 0) even++;
  else odd++;
}

System.out.println("Digits: " + count);
System.out.println("Sum: " + sum);
System.out.println("Even digits: " + even);
System.out.println("Odd digits: " + odd);

🔍 Step-by-step:

• % 10 extracts the last digit
• /= 10 removes the last digit
• if (digit % 2 == 0) checks for even

🔁 Using while loop

int number = 4623;
int original = number;
int count = 0, even = 0, odd = 0, sum = 0;

while (number > 0) {
  int digit = number % 10;
  count++;
  sum += digit;
  if (digit % 2 == 0) even++;
  else odd++;
  number /= 10;
}

System.out.println("Digits: " + count);
System.out.println("Sum: " + sum);
System.out.println("Even digits: " + even);
System.out.println("Odd digits: " + odd);

🔂 Using do-while loop

int number = 4623;
int original = number;
int count = 0, even = 0, odd = 0, sum = 0;

do {
  int digit = number % 10;
  count++;
  sum += digit;
  if (digit % 2 == 0) even++;
  else odd++;
  number /= 10;
} while (number > 0);

System.out.println("Digits: " + count);
System.out.println("Sum: " + sum);
System.out.println("Even digits: " + even);
System.out.println("Odd digits: " + odd);

✅ Sample Output

Digits: 4
Sum: 15
Even digits: 3
Odd digits: 1

🧠 Which loop is best?

Loop	Best When...
for	You know all parts: start, end, and update
while	You only need to repeat based on a condition
do-while	You want the loop to run at least once

💡 Armstrong Number Logic in Java

An Armstrong number is a number that is equal to the sum of the cubes of its digits. For example, 153 is an Armstrong number because:

1³ + 5³ + 3³ = 1 + 125 + 27 = 153

✅ Java Code Example

public class ArmstrongCheck {
  public static void main(String[] args) {
    int number = 153;
    int originalNumber = number;
    int sum = 0;

    while (number > 0) {
      int digit = number % 10;
      sum += digit * digit * digit;
      number /= 10;
    }

    if (sum == originalNumber) {
      System.out.println(originalNumber + " is an Armstrong number.");
    } else {
      System.out.println(originalNumber + " is not an Armstrong number.");
    }
  }
}

🔍 Key Concepts

• number % 10: Extracts the last digit.
• digit * digit * digit: Cubes the digit.
• number /= 10: Removes the last digit (integer division).

❓ Why Use while (number > 0) Instead of while (number != 0)?

• ✔️ Clarity: Clearly expresses intent to process positive digits only.
• ✔️ Safety: Prevents infinite loops if a negative number is entered.
• ❌ Risk with != 0: If number is negative, it never becomes 0 — leads to infinite loop.

📊 Comparison

Condition	Works for Positive?	Works for Zero?	Works for Negative?	Issue
while (number > 0)	✅ Yes	✅ Yes (loop doesn't run)	✅ Yes (loop doesn't run)	None
while (number != 0)	✅ Yes	✅ Yes (loop doesn't run)	❌ No	Infinite loop for negative input

📝 Final Tip

Use the while (number > 0) condition when processing digits of a number. It ensures your loop is safe, predictable, and avoids edge-case bugs from negative inputs.

🔁 Palindrome Number Check in Java

A palindrome number is a number that remains the same when its digits are reversed. For example, 121, 1331, and 454 are palindromes.

✅ Java Code Example

public class PalindromeCheck {
  public static void main(String[] args) {
    int number = 121;
    int originalNumber = number;
    int reversed = 0;

    while (number > 0) {
      int digit = number % 10;
      reversed = reversed * 10 + digit;
      number /= 10;
    }

    if (reversed == originalNumber) {
      System.out.println(originalNumber + " is a palindrome.");
    } else {
      System.out.println(originalNumber + " is not a palindrome.");
    }
  }
}

🔍 How It Works

• number % 10: Extracts the last digit.
• reversed = reversed * 10 + digit: Builds the reversed number.
• number /= 10: Removes the last digit.

❓ Why Use while (number > 0) and Not while (number != 0)?

• ✔️ Prevents infinite loop: number != 0 can cause infinite loop for negative input.
• ✔️ Palindromes are non-negative by definition: So number > 0 safely skips negatives.
• ✔️ Better readability: Shows intention to loop through digits until the number is 0.

📊 Condition Comparison

Condition	Positive Input	Zero Input	Negative Input	Potential Issue
while (number > 0)	✅ Yes	✅ Yes (loop doesn't run)	✅ Yes (loop doesn't run)	None
while (number != 0)	✅ Yes	✅ Yes (loop doesn't run)	❌ No	Infinite loop for negative input

📝 Final Tip

Use while (number > 0) to safely reverse digits when checking for palindromes. It's more robust and avoids accidental infinite loops from negative numbers.

⭐ Right-angled triangle Star Pattern in Java Using All Loop Types

Let's print a right-angled triangle star pattern using for, while, and do-while loops in Java. Each loop gives the same output but works differently in structure.

✅ 1. Using for Loop

public class StarPatternFor {
  public static void main(String[] args) {
    for (int i = 1; i <= 5; i++) {
      for (int j = 1; j <= i; j++) {
        System.out.print("* ");
      }
      System.out.println();
    }
  }
}

✅ 2. Using while Loop

public class StarPatternWhile {
  public static void main(String[] args) {
    int i = 1;
    while (i <= 5) {
      int j = 1;
      while (j <= i) {
        System.out.print("* ");
        j++;
      }
      System.out.println();
      i++;
    }
  }
}

✅ 3. Using do-while Loop

public class StarPatternDoWhile {
  public static void main(String[] args) {
    int i = 1;
    do {
      int j = 1;
      do {
        System.out.print("* ");
        j++;
      } while (j <= i);
      System.out.println();
      i++;
    } while (i <= 5);
  }
}

🧠 Pattern Logic

• Each row i has i stars.
• Outer loop tracks the row number (1 to 5).
• Inner loop prints stars based on the current row.

🔄 Loop Comparison

Loop Type	Entry Check?	Common Usage
for	✅ Condition checked before loop	Best when number of iterations is known
while	✅ Condition checked before loop	Use when you may not know iterations in advance
do-while	✅ Condition checked after loop	Ensures loop runs at least once

📌 Output

* 
* * 
* * * 
* * * * 
* * * * * 

📝 Final Tip

Use for loop when the size of the pattern is fixed. Use while or do-while for more dynamic or user-driven patterns, especially when input is taken at runtime.

🌟 Star Patterns in Java (All Loops)

📐 Pattern 1: Right-Angled Triangle

*
* *
* * *
* * * *
* * * * *

✅ For Loop

for (int i = 1; i <= 5; i++) {
  for (int j = 1; j <= i; j++) {
    System.out.print("* ");
  }
  System.out.println();
}

✅ While Loop

int i = 1;
while (i <= 5) {
  int j = 1;
  while (j <= i) {
    System.out.print("* ");
    j++;
  }
  System.out.println();
  i++;
}

✅ Do-While Loop

int i = 1;
do {
  int j = 1;
  do {
    System.out.print("* ");
    j++;
  } while (j <= i);
  System.out.println();
  i++;
} while (i <= 5);

📐 Pattern 2: Inverted Triangle

* * * * *
* * * *
* * *
* *
*

✅ For Loop

for (int i = 5; i >= 1; i--) {
  for (int j = 1; j <= i; j++) {
    System.out.print("* ");
  }
  System.out.println();
}

✅ While Loop

int i = 5;
while (i >= 1) {
  int j = 1;
  while (j <= i) {
    System.out.print("* ");
    j++;
  }
  System.out.println();
  i--;
}

✅ Do-While Loop

int i = 5;
do {
  int j = 1;
  do {
    System.out.print("* ");
    j++;
  } while (j <= i);
  System.out.println();
  i--;
} while (i >= 1);

📐 Pattern 3: Pyramid

    *
   * *
  * * *
 * * * *
* * * * *

✅ For Loop

for (int i = 1; i <= 5; i++) {
  for (int space = 1; space <= 5 - i; space++) {
    System.out.print(" ");
  }
  for (int j = 1; j <= i; j++) {
    System.out.print("* ");
  }
  System.out.println();
}

📐 Pattern 4: Inverted Pyramid

* * * * *
 * * * *
  * * *
   * *
    *

✅ For Loop

for (int i = 5; i >= 1; i--) {
  for (int space = 1; space <= 5 - i; space++) {
    System.out.print(" ");
  }
  for (int j = 1; j <= i; j++) {
    System.out.print("* ");
  }
  System.out.println();
}

📐 Pattern 5: Diamond

    *
   * *
  * * *
 * * * *
* * * * *
 * * * *
  * * *
   * *
    *

✅ For Loop (Full Diamond)

// Upper Half
for (int i = 1; i <= 5; i++) {
  for (int space = 1; space <= 5 - i; space++) {
    System.out.print(" ");
  }
  for (int j = 1; j <= i; j++) {
    System.out.print("* ");
  }
  System.out.println();
}
// Lower Half
for (int i = 4; i >= 1; i--) {
  for (int space = 1; space <= 5 - i; space++) {
    System.out.print(" ");
  }
  for (int j = 1; j <= i; j++) {
    System.out.print("* ");
  }
  System.out.println();
}

📌 Tip:

• Use nested loops: outer for rows, inner for spaces or stars.
• Play with spacing to align the patterns correctly.
• Pattern size can be user input (e.g., replace 5 with n).

📐 Number Patterns in Java with Explanations

1️⃣ Right-Angled Number Triangle (For Loop)

Print numbers in a right-angled triangle using nested for loops.

int n = 5;
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= i; j++) {
        System.out.print(j + " ");
    }
    System.out.println();
}

• Outer loop (i) runs from 1 to n → number of rows.
• Inner loop (j) runs from 1 to i → print increasing numbers per row.

2️⃣ Inverted Number Triangle (For Loop)

Print decreasing sequences in each row.

int n = 5;
for (int i = n; i >= 1; i--) {
    for (int j = 1; j <= i; j++) {
        System.out.print(j + " ");
    }
    System.out.println();
}

• Outer loop runs from n to 1.
• Inner loop prints from 1 to current row limit.

3️⃣ Number Pyramid (For Loop)

Create a centered pyramid and its inverted version using numbers.

🔼 Number Pyramid

int n = 5;
for (int i = 1; i <= n; i++) {
    // Print leading spaces
    for (int s = 1; s <= n - i; s++) {
        System.out.print(" ");
    }
    // Print numbers with space
    for (int j = 1; j <= i; j++) {
        System.out.print(j + " ");
    }
    System.out.println();
}

🔹 Output:

    1
   1 2
  1 2 3
 1 2 3 4
1 2 3 4 5

🔻 Inverted Number Pyramid

int n = 5;
for (int i = n; i >= 1; i--) {
    // Print leading spaces
    for (int s = 1; s <= n - i; s++) {
        System.out.print(" ");
    }
    // Print numbers with space
    for (int j = 1; j <= i; j++) {
        System.out.print(j + " ");
    }
    System.out.println();
}

🔹 Output:

1 2 3 4 5
 1 2 3 4
  1 2 3
   1 2
    1

💡 Notes:

• Leading spaces align the numbers in a triangle shape.
• System.out.print(j + " "); adds a number and a space for proper width.
• The structure is built using nested loops: outer loop for rows, inner loops for spaces and numbers.

4️⃣ Diamond Number Pattern (For Loop)

Upper and lower pyramid combined to form a diamond with numbers.

int n = 5;

// Upper half
for (int i = 1; i <= n; i++) {
    for (int s = 1; s <= n - i; s++) {
        System.out.print(" ");
    }
    for (int j = 1; j <= i; j++) {
        System.out.print(j + " ");
    }
    System.out.println();
}

// Lower half
for (int i = n - 1; i >= 1; i--) {
    for (int s = 1; s <= n - i; s++) {
        System.out.print(" ");
    }
    for (int j = 1; j <= i; j++) {
        System.out.print(j + " ");
    }
    System.out.println();
}

• Two parts: upper pyramid and inverted pyramid.
• Each line starts with spaces to center the numbers.
• Numbers increase from 1 to i on each line.

📌 Output for n = 5:

    1 
   1 2 
  1 2 3 
 1 2 3 4 
1 2 3 4 5 
 1 2 3 4 
  1 2 3 
   1 2 
    1

5️⃣ Floyd's Triangle

Numbers increase sequentially row by row.

int n = 5;
int num = 1;

for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= i; j++) {
        System.out.print(num + " ");
        num++;
    }
    System.out.println();
}

• num starts at 1 and increases with each printed number.
• Each row i contains exactly i numbers.
• Forms a triangular pattern from top to bottom.

📌 Output for n = 5:

1
2 3
4 5 6
7 8 9 10
11 12 13 14 15

📘 Non-Primitive Data Type: String

In Java, a String is a non-primitive data type that represents a sequence of characters. It is an object of the String class and provides various useful methods for manipulation.

🔹 String Basics

// Declaration
String name = "Aelify";

// Common operations
System.out.println(name.length());      // 6
System.out.println(name.charAt(0));     // 'A'
System.out.println(name.toUpperCase()); // "AELIFY"
System.out.println(name.substring(1));  // "elify"

🧠 Concept: Loop Through a String

Let's print each character in a string using all 3 loops.

▶️ Using for loop

String str = "Java";
for (int i = 0; i < str.length(); i++) {
    System.out.println(str.charAt(i));
}

▶️ Using while loop

String str = "Java";
int i = 0;
while (i < str.length()) {
    System.out.println(str.charAt(i));
    i++;
}

▶️ Using do-while loop

String str = "Java";
int i = 0;
do {
    System.out.println(str.charAt(i));
    i++;
} while (i < str.length());

🔍 Step-by-Step Explanation

1. Declare a String variable with a value, e.g., "Java".
2. Use a loop to access each index of the string using charAt(i).
3. Print each character on a new line.

📌 Output for all loops (str = "Java")

J
a
v
a

✅ Recap

• String is an object that represents character sequences.
• It is immutable - once created, it cannot be changed.
• You can use for, while, or do-while to iterate over each character.

📦 Java Classes — Non-Primitive Data Type

In Java, a class is a blueprint for creating objects. It encapsulates data for the object and methods to manipulate that data. Let's understand how classes work with a step-by-step explanation and looping usage.

🧒 Simple Explanation: What Is a Class in Java?

Think of a class like a blueprint or a template for something real — like a Student, Car, or Animal.

• 🧱 A class tells Java what things (data) an object should have, and what it can do (methods).
• 👶 An object is made from a class — like creating a real student from the student template.
• 🚪 A constructor is like a door that lets you create objects with specific values.
• 🗣 A method is like a behavior — it tells the object what to do (like speak or display info).

📦 In Simple Words:

Imagine you're making a toy using a mold (class). Every time you pour plastic into the mold (create an object), you get a new toy with the shape and features you designed.

🔄 Steps to Create and Use a Class in Java

1. Create the class: Define the data (like name and age).
2. Add a constructor: This helps you set values when the object is created.
3. Add methods: These perform actions, like showing the info.
4. Use the class in main(): Make real students using new keyword.
5. Use loops: If you have many students, use for, while, or do-while to show all their info.

✅ Example in Real Life:

A class is like a cookie-cutter 🍪. You design it once, and then make many cookies (objects) from it. Each cookie can have different flavors (data).

🧠 Tip:

Always start by asking: "What do I want to model?" — That will become your class.

✅ Basic Class Structure

class Student {
  String name;
  int age;

  // Constructor
  Student(String name, int age) {
    this.name = name;
    this.age = age;
  }

  // Method
  void displayInfo() {
    System.out.println("Name: " + name + ", Age: " + age);
  }
}

🧪 Example: Using Class in Main Method

class Student {
  String name;
  int age;

  Student(String name, int age) {
    this.name = name;
    this.age = age;
  }

  void displayInfo() {
    System.out.println("Name: " + name + ", Age: " + age);
  }
}

public class Main {
  public static void main(String[] args) {
    Student s1 = new Student("Aelify", 20);
    Student s2 = new Student("Bob", 22);

    s1.displayInfo();
    s2.displayInfo();
  }
}

🔁 Looping Over Class Objects (Array of Objects)

You can use loops to handle multiple class objects efficiently.

📌 Using for loop

class Student {
  String name;
  int age;

  Student(String name, int age) {
    this.name = name;
    this.age = age;
  }

  void displayInfo() {
    System.out.println("Name: " + name + ", Age: " + age);
  }
}

public class Main {
  public static void main(String[] args) {
    Student[] students = {
      new Student("Aelify", 20),
      new Student("Bob", 21),
      new Student("Charlie", 22)
    };

    for (int i = 0; i < students.length; i++) {
      students[i].displayInfo();
    }
  }
}

📌 Using while loop

class Student {
  String name;
  int age;

  Student(String name, int age) {
    this.name = name;
    this.age = age;
  }

  void displayInfo() {
    System.out.println("Name: " + name + ", Age: " + age);
  }
}

public class Main {
  public static void main(String[] args) {
    Student[] students = {
      new Student("Aelify", 20),
      new Student("Bob", 21),
      new Student("Charlie", 22)
    };

    int i = 0;
    while (i < students.length) {
      students[i].displayInfo();
      i++;
    }
  }
}

📌 Using do-while loop

class Student {
  String name;
  int age;

  Student(String name, int age) {
    this.name = name;
    this.age = age;
  }

  void displayInfo() {
    System.out.println("Name: " + name + ", Age: " + age);
  }
}

public class Main {
  public static void main(String[] args) {
    Student[] students = {
      new Student("Aelify", 20),
      new Student("Bob", 21),
      new Student("Charlie", 22)
    };

    int i = 0;
    do {
      students[i].displayInfo();
      i++;
    } while (i < students.length);
  }
}

🔍 Step-by-Step Breakdown

• ✅ Create a class — Define attributes and behaviors using variables and methods.
• ✅ Constructor — Used to initialize objects with specific values.
• ✅ Object creation — Use new keyword to instantiate the class.
• ✅ Looping — Store objects in an array and loop through them for bulk operations.

📌 Output Example

Name: Aelify, Age: 20
Name: Bob, Age: 21
Name: Charlie, Age: 22

✅ Arrays in Java

Arrays are non-primitive data structures that store multiple values of the same type in a single variable. They are indexed starting from 0.

An array is like a row of boxes 🧱. Each box stores a value, and we can find any value by its box number (called an index). Indexes start from 0.

📦 Array Declaration & Initialization

public class Main {
  public static void main(String[] args) {
    int[] numbers = {10, 20, 30, 40, 50};

    for (int i = 0; i < numbers.length; i++) {
      System.out.println("Element at index " + i + ": " + numbers[i]);
    }
  }
}

🔁 Loop using while

public class Main {
  public static void main(String[] args) {
    int[] numbers = {10, 20, 30, 40, 50};

    int i = 0;
    while (i < numbers.length) {
      System.out.println("Element at index " + i + ": " + numbers[i]);
      i++;
    }
  }
}

🔂 Loop using do-while

public class Main {
  public static void main(String[] args) {
    int[] numbers = {10, 20, 30, 40, 50};

    int j = 0;
    do {
      System.out.println("Element at index " + j + ": " + numbers[j]);
      j++;
    } while (j < numbers.length);
  }
}

➕ Sum of Array Elements

public class Main {
  public static void main(String[] args) {
    int[] numbers = {10, 20, 30, 40, 50};

    int sum = 0;
    for (int num : numbers) {
      sum += num;
    }

    System.out.println("Sum of elements: " + sum);
  }
}

🧠 Multi-dimensional Array (2D)

public class Main {
  public static void main(String[] args) {
    int[][] matrix = {
      {1, 2},
      {3, 4},
      {5, 6}
    };

    for (int row = 0; row < matrix.length; row++) {
      for (int col = 0; col < matrix[row].length; col++) {
        System.out.print(matrix[row][col] + " ");
      }
      System.out.println();
    }
  }
}

📘 Simple Explanation (for beginners)

Imagine an array like a row of mailboxes 📬. Each one holds a number and has a label: 0, 1, 2... etc.

• Declare: Tell Java "I'm making an array of numbers" → int[] numbers;
• Initialize: Fill it with values → {10, 20, 30, 40, 50}
• Access: Use numbers[0] to see what’s inside the first box
• Loop: Let Java open all boxes one by one using a for or while loop
• Sum: Add up all numbers using a loop

🎒 Real-Life Analogy

Think of an array like a row of school bags. Each bag has a number tag (index) and holds a book (value). You can loop through the bags to find what’s inside!

An array is like a train 🚆 with multiple coaches. Each coach carries a passenger (number). You can walk through the coaches one by one to check who’s sitting where!

✅ Output Example

Element at index 0: 10
Element at index 1: 20
Element at index 2: 30
Element at index 3: 40
Element at index 4: 50
Sum of elements: 150

🧠 Java Matrices (Multi-dimensional Arrays)

A matrix in Java is essentially a 2D array — an array of arrays. Think of it like a table with rows and columns, where each cell holds a value.

🔹 Declaring a 2D Array (Matrix)

int[][] matrix = new int[3][2]; // 3 rows, 2 columns

This creates a matrix like:

[0][0] [0][1]
[1][0] [1][1]
[2][0] [2][1]

public class Main {
  public static void main(String[] args) {
    // Declares a 2D array (matrix) with 3 rows and 2 columns
    int[][] matrix = new int[3][2];

    // Outer loop iterates over each row
    for (int i = 0; i < matrix.length; i++) {
      // Inner loop iterates over each column in the current row
      for (int j = 0; j < matrix[i].length; j++) {
        // Assigns the value i + j to each element in the matrix
        matrix[i][j] = i + j;
        // Prints the current element followed by a space
        System.out.print(matrix[i][j] + " ");
      }
      // Moves to the next line after printing one row
      System.out.println();
    }
  }
}

🔹 Initializing a Matrix Directly

public class Main {
  public static void main(String[] args) {
    // Declares and initializes a 2D array with 3 rows and 2 columns
    int[][] matrix = {
      {1, 2},
      {3, 4},
      {5, 6}
    };

    // Traverses and prints each element using nested loops
    for (int i = 0; i < matrix.length; i++) {
      for (int j = 0; j < matrix[i].length; j++) {
        // Prints each element followed by a space
        System.out.print(matrix[i][j] + " ");
      }
      // Moves to the next line after each row
      System.out.println();
    }
  }
}

🔍 Accessing Elements

public class Main {
  public static void main(String[] args) {
    // Initializes a 2D array with fixed values
    int[][] matrix = {
      {1, 2},
      {3, 4},
      {5, 6}
    };

    // Accesses and prints the element in the first row, second column
    System.out.println("Element at [0][1]: " + matrix[0][1]); // prints 2
  }
}

🔁 Traversing a Matrix with Nested Loops

public class Main {
  public static void main(String[] args) {
    // Initializes a 2D array with 3 rows and 2 columns
    int[][] matrix = {
      {1, 2},
      {3, 4},
      {5, 6}
    };

    // Outer loop goes through each row
    for (int row = 0; row < matrix.length; row++) {
      // Inner loop goes through each column in the current row
      for (int col = 0; col < matrix[row].length; col++) {
        // Prints each element followed by a space
        System.out.print(matrix[row][col] + " ");
      }
      // Moves to next line after printing a full row
      System.out.println();
    }
  }
}

🌟 Enhanced For Loop (for-each)

public class Main {
  public static void main(String[] args) {
    // Initializes a 2D array
    int[][] matrix = {
      {1, 2},
      {3, 4},
      {5, 6}
    };

    // For-each loop to go through each row (which is a 1D array)
    for (int[] row : matrix) {
      // For-each loop to go through each element in the current row
      for (int num : row) {
        // Prints each number with a space
        System.out.print(num + " ");
      }
      // Moves to the next line after printing a row
      System.out.println();
    }
  }
}

➕ Matrix Addition

public class Main {
  public static void main(String[] args) {
    // Declares and initializes two 2x2 matrices A and B
    int[][] A = {
      {1, 2},
      {3, 4}
    };
    int[][] B = {
      {5, 6},
      {7, 8}
    };

    // Creates a result matrix to store the sum, same size as A and B
    int[][] result = new int[2][2];

    // Loops over each element to add corresponding elements of A and B
    for (int i = 0; i < A.length; i++) {
      for (int j = 0; j < A[0].length; j++) {
        result[i][j] = A[i][j] + B[i][j];
      }
    }

    // Prints the resulting matrix
    for (int[] row : result) {
      for (int value : row) {
        System.out.print(value + " ");
      }
      System.out.println();
    }
  }
}

✖️ Matrix Multiplication

public class Main {
  public static void main(String[] args) {
    // Declares and initializes matrices A and B
    int[][] A = {
      {1, 2},
      {3, 4}
    };
    int[][] B = {
      {2, 0},
      {1, 2}
    };

    // Resultant matrix for storing multiplication results
    int[][] product = new int[2][2];

    // Triple nested loop to perform matrix multiplication
    for (int i = 0; i < 2; i++) {
      for (int j = 0; j < 2; j++) {
        // Initialize current cell to 0
        product[i][j] = 0;
        // Perform the dot product of row of A and column of B
        for (int k = 0; k < 2; k++) {
          product[i][j] += A[i][k] * B[k][j];
        }
      }
    }

    // Prints the product matrix
    for (int[] row : product) {
      for (int value : row) {
        System.out.print(value + " ");
      }
      System.out.println();
    }
  }
}

📏 Transpose of a Matrix

public class Main {
  public static void main(String[] args) {
    // Original matrix with 3 rows and 2 columns
    int[][] original = {
      {1, 2},
      {3, 4},
      {5, 6}
    };

    // Transpose matrix will have 2 rows and 3 columns
    int[][] transpose = new int[2][3];

    // Loops through each element and swaps rows with columns
    for (int i = 0; i < original.length; i++) {
      for (int j = 0; j < original[0].length; j++) {
        transpose[j][i] = original[i][j];
      }
    }

    // Prints the transposed matrix
    for (int[] row : transpose) {
      for (int value : row) {
        System.out.print(value + " ");
      }
      System.out.println();
    }
  }
}

🧠 Advanced: Jagged Arrays

These are arrays where each row can have a different number of columns:

public class Main {
  public static void main(String[] args) {
    // Declares a jagged array with 3 rows and unspecified column sizes
    int[][] jagged = new int[3][];
    // First row has 2 elements
    jagged[0] = new int[]{1, 2};
    // Second row has 3 elements
    jagged[1] = new int[]{3, 4, 5};
    // Third row has 1 element
    jagged[2] = new int[]{6};

    // Loops through each row
    for (int i = 0; i < jagged.length; i++) {
      // Loops through each column in current row
      for (int j = 0; j < jagged[i].length; j++) {
        // Prints each element followed by space
        System.out.print(jagged[i][j] + " ");
      }
      // New line after each row
      System.out.println();
    }
  }
}

✅ Recap:

• 2D Arrays = Tables of data (rows & columns)
• Access using matrix[i][j]
• Use nested loops to traverse
• Common tasks: Addition, Multiplication, Transpose
• Jagged arrays allow flexible row sizes

✅ With these tools, you can now build anything from a spreadsheet to a tic-tac-toe game using matrices!

🧠 Java Matrices (2D Arrays)

Matrices (2D arrays) are like tables in Java — rows and columns of data. They're used in everything from spreadsheets to games like tic-tac-toe.

🎮 Project 1: Tic-Tac-Toe Game

import java.util.Scanner;

public class TicTacToe {
  public static void main(String[] args) {
    // Initialize the board with 3x3 empty spaces
    char[][] board = {
      {' ', ' ', ' '},
      {' ', ' ', ' '},
      {' ', ' ', ' '}
    };

    Scanner sc = new Scanner(System.in);
    char currentPlayer = 'X';

    for (int moves = 0; moves < 9; moves++) {
      // Print current board
      System.out.println("\nCurrent Board:");
      for (char[] row : board) {
        for (char cell : row) {
          System.out.print("|" + cell);
        }
        System.out.println("|");
      }

      System.out.println("Player " + currentPlayer + ", enter row and column (0-2):");
      int row = sc.nextInt();
      int col = sc.nextInt();

      // ✅ Check for invalid input range
      if (row < 0 || row > 2 || col < 0 || col > 2) {
        System.out.println("❌ Invalid input! Row and column must be between 0 and 2. Try again.");
        moves--; // Don't count this as a valid move
        continue; // Ask again
      }

      // ✅ Check if cell is already occupied
      if (board[row][col] == ' ') {
        board[row][col] = currentPlayer;

        // ✅ Check for winning conditions
        if ((board[row][0] == currentPlayer && board[row][1] == currentPlayer && board[row][2] == currentPlayer) ||
            (board[0][col] == currentPlayer && board[1][col] == currentPlayer && board[2][col] == currentPlayer) ||
            (board[0][0] == currentPlayer && board[1][1] == currentPlayer && board[2][2] == currentPlayer) ||
            (board[0][2] == currentPlayer && board[1][1] == currentPlayer && board[2][0] == currentPlayer)) {
          System.out.println("🎉 Player " + currentPlayer + " wins!");
          return;
        }

        // Switch player
        currentPlayer = (currentPlayer == 'X') ? 'O' : 'X';
      } else {
        System.out.println("❌ Cell already taken! Try again.");
        moves--; // Don't count this move
      }
    }

    System.out.println("🤝 It's a draw!");
  }
}

📊 Project 2: Simple Spreadsheet

import java.util.Scanner;

public class Spreadsheet {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

    // Ask user for number of rows and columns
    System.out.print("Enter number of rows: ");
    int rows = scanner.nextInt();

    System.out.print("Enter number of columns: ");
    int cols = scanner.nextInt();

    // Create a 2D array with user-defined size
    int[][] data = new int[rows][cols];

    // 🔹 Input Spreadsheet Data
    System.out.println("\nEnter values for the spreadsheet:");
    for (int i = 0; i < rows; i++) {
      for (int j = 0; j < cols; j++) {
        System.out.print("Enter value for cell [" + i + "][" + j + "]: ");
        data[i][j] = scanner.nextInt();
      }
    }

    System.out.println();

    // 🔹 Sum of Each Row
    System.out.println("📌 Sum of Each Row:");
    for (int i = 0; i < rows; i++) {
      int rowSum = 0;
      for (int j = 0; j < cols; j++) {
        rowSum += data[i][j];
      }
      System.out.println("Sum of row " + i + ": " + rowSum);
    }

    System.out.println();

    // 🔹 Sum of Each Column
    System.out.println("📌 Sum of Each Column:");
    for (int col = 0; col < cols; col++) {
      int colSum = 0;
      for (int row = 0; row < rows; row++) {
        colSum += data[row][col];
      }
      System.out.println("Sum of column " + col + ": " + colSum);
    }

    System.out.println();

    // 🔹 Total Sum of All Elements
    int totalSum = 0;
    for (int i = 0; i < rows; i++) {
      for (int j = 0; j < cols; j++) {
        totalSum += data[i][j];
      }
    }
    System.out.println("🧮 Total sum of all elements: " + totalSum);

    System.out.println();

    // 🔹 Display Spreadsheet
    System.out.println("📋 Spreadsheet View:");
    for (int[] row : data) {
      for (int val : row) {
        System.out.print(val + "\t");
      }
      System.out.println();
    }

    scanner.close(); // Clean up
  }
}

🐍 Snake Game in Java (Console-based)

🐍 This is a simple console-based Snake Game in Java designed for absolute beginners! It uses a 2D array (matrix) to simulate a game board where a snake ('S') moves using WASD keys and tries to eat food ('F') randomly placed on the grid. Each time the snake eats the food, your score increases, and new food appears elsewhere on the board. The game ends if you hit the wall or choose to quit. 🚧

🎯 What You'll Learn:

• How to use 2D arrays (matrices) in Java
• How to take keyboard input using Scanner
• How to generate random positions with Random
• Basic game loop structure and state handling
• How to check for collisions (like wall boundaries)
• How to update a visual game board in the console

🧩 Concept

• Matrix size: 10x10
• Snake: represented by 'S'
• Food: represented by 'F'
• Empty cell: '.'

🧠 Logic

• Create a 10x10 2D array to represent the game board.
• Initialize all cells to '.' (empty space).
• Place the snake's starting position at the center using 'S'.
• Randomly place food on an empty cell using 'F'.
• Start a game loop that runs while the game is active.
• Use Scanner to read player input: W (up), A (left), S (down), D (right), Q (quit).
• Before moving, clear the snake's previous cell by setting it back to '.'.
• Update the snake’s position based on the input direction.
• Check for wall collisions: if the snake moves outside the board, end the game.
• If the snake moves onto a cell with food, increase the score and generate new food in a random empty cell.
• Update the new snake position on the board with 'S'.

🔤 Code (SnakeGame.java)

// 📦 Importing utility classes needed for random number generation and user input
import java.util.Random;  // 🎲 Used to generate random positions for food on the board
import java.util.Scanner; // ⌨️ Used to read user keyboard input (W, A, S, D, Q)

public class SnakeGame {
    public static void main(String[] args) {
        // 🎯 Set dimensions for the game board (10 rows x 10 columns)
        int rows = 10;
        int cols = 10;

        // 🧱 Create a 2D array (matrix) to represent the board
        // Each cell can be: '.' (empty), 'S' (snake), or 'F' (food)
        char[][] board = new char[rows][cols];

        // 🐍 Initialize snake's starting position at the center of the board
        int snakeRow = rows / 2;  // Row index for snake's position
        int snakeCol = cols / 2;  // Column index for snake's position

        // 🍎 Variables to hold the food's position
        int foodRow, foodCol;

        // 🎮 Game tracking variables
        int score = 0;             // Keeps track of how much food the snake eats
        boolean isRunning = true;  // Controls whether the game loop keeps running

        // ⌨️ Create a Scanner object to read keyboard input from the user
        Scanner scanner = new Scanner(System.in);

        // 🎲 Create a Random object to generate random food positions
        Random random = new Random();

        // 🔁 Fill the entire board with '.' to represent empty spaces
        for (int i = 0; i < rows; i++) {        // Loop through each row
            for (int j = 0; j < cols; j++) {    // Loop through each column in the current row
                board[i][j] = '.';              // Set cell to empty
            }
        }

        // 🐍 Place the snake's starting position on the board
        board[snakeRow][snakeCol] = 'S';

        // 🍎 Place the food at a random position that is NOT on the snake
        do {
            foodRow = random.nextInt(rows); // Random row between 0 and rows-1
            foodCol = random.nextInt(cols); // Random column between 0 and cols-1
        } while (board[foodRow][foodCol] == 'S'); // Repeat if it lands on the snake

        // 🧃 Place the food symbol on the board
        board[foodRow][foodCol] = 'F';

        // 🕹️ Main game loop — runs until player quits or hits a wall
        while (isRunning) {
            // 💬 Display the current score to the player
            System.out.println("\nScore: " + score);

            // 🖨️ Print column numbers at the top of the board
            System.out.print("   "); // Extra spaces to align columns with row numbers
            for (int j = 0; j < cols; j++) {
                System.out.print(j + " "); // Print column index
            }
            System.out.println(); // Move to next line

            // 🖨️ Print the board row by row with row numbers on the left
            for (int i = 0; i < rows; i++) {
                System.out.print(i + "  "); // Print row index with spacing
                for (int j = 0; j < cols; j++) {
                    System.out.print(board[i][j] + " "); // Print cell content
                }
                System.out.println(); // Move to next row
            }

            // ⌨️ Ask player for movement input
            System.out.print("Move (WASD to move, Q to quit): ");
            String input = scanner.nextLine().toUpperCase(); // Read and convert to uppercase

            // ✅ Prepare to check the move
            boolean validMove = true; // Tracks if input is a valid direction
            int newRow = snakeRow;    // Temporary new row position for snake
            int newCol = snakeCol;    // Temporary new column position for snake

            // 🧭 Decide the direction based on input
            switch (input) {
                case "W": newRow--; break; // Move up
                case "A": newCol--; break; // Move left
                case "S": newRow++; break; // Move down
                case "D": newCol++; break; // Move right
                case "Q":                  // Quit the game
                    isRunning = false;     // Stop game loop
                    continue;              // Skip rest of this loop iteration
                default:
                    // ❌ Invalid input — warn player and keep snake where it is
                    System.out.println("Invalid input! Use W, A, S, D to move.");
                    validMove = false; // Mark the move as invalid
                    break;
            }

            // 🚫 If invalid move, skip updating the snake and redraw the board
            if (!validMove) {
                continue; // Go back to top of loop
            }

            // 🚧 Check if new position is outside the board (wall collision)
            if (newRow < 0 || newRow >= rows || newCol < 0 || newCol >= cols) {
                System.out.println("💥 Game Over! You hit the wall.");
                break; // End the game loop
            }

            // 🧽 Clear the snake's old position (replace with '.')
            board[snakeRow][snakeCol] = '.';

            // 🔄 Update snake position to new coordinates
            snakeRow = newRow;
            snakeCol = newCol;

            // 🍽️ Check if snake moved into the food's position
            if (snakeRow == foodRow && snakeCol == foodCol) {
                score++; // Increase score

                // 🍎 Place new food somewhere not on the snake
                do {
                    foodRow = random.nextInt(rows); // Random new food row
                    foodCol = random.nextInt(cols); // Random new food col
                } while (board[foodRow][foodCol] == 'S'); // Avoid snake position

                // 🧃 Place new food on the board
                board[foodRow][foodCol] = 'F';
            }

            // 🐍 Place snake's new position on the board
            board[snakeRow][snakeCol] = 'S';
        }

        // 🧹 Close scanner to release system resources
        scanner.close();

        // 🎉 Show the player's final score after game ends
        System.out.println("👋 Game Ended. Final Score: " + score);
    }
}

📌 Output Sample

Score: 2
   0 1 2 3 4 5 6 7 8 9
0  . . . . . . . . . .
1  . . . . . . . . . .
2  . S . . . . . . . .
3  . . . . . . . F . .
4  . . . . . . . . . .
5  . . . . . . . . . .
6  . . . . . . . . . .
7  . . . . . . . . . .
8  . . . . . . . . . .
9  . . . . . . . . . .
Move (WASD to move, Q to quit):

🧠 Key Changes & Explanation:

Feature	Explanation
Food (F)	Randomly placed on any empty cell on the board. If snake eats it, score increases.
Score	An integer that increases each time food is eaten.
Game End	If snake moves out of bounds or player quits (Q).
Random Food	Re-positioned after each eating, not allowed on the snake cell.

🧠 Concepts Used in This Game:

Concept	Explanation
2D Arrays	Used to represent the game board (char[][] board) as a grid.
Scanner	Reads user input from the keyboard.
Random	Picks random positions to place the food.
Loops	Used to fill and print the board, and repeat the game until quitting.
Conditions	Used to check user input and whether the snake hits the wall or eats the food.
Switch	Clean way to handle movement options.
Functions	Main logic is inside main, split with comments into logical steps.

🧯 Common Mistakes to Avoid

• Forgetting to import required classes:
  If you forget import java.util.Random; or import java.util.Scanner;, your program won’t compile. Java won’t recognize Random or Scanner without these imports.
• Not closing the Scanner:
  Always close the Scanner object with scanner.close() at the end of the program to free system resources.
• Creating multiple Scanner instances for the same input stream:
  Using more than one Scanner for System.in can cause unexpected behavior or skipped inputs. Reuse the same instance throughout the program.
• Using Random incorrectly:
  Calling random.nextInt() without an argument will throw an error. Always specify the bound, e.g., random.nextInt(rows) to stay within the board range.
• Hardcoding positions instead of using variables:
  Avoid using fixed numbers for board positions. Always calculate positions dynamically using rows, cols, snakeRow, and snakeCol.

✅ What You Learned

• How to import and use Java utility classes like Random (for random numbers) and Scanner (for user input)
• How to set up and manage game variables (board size, score, and a boolean to control the game loop)
• How to create and work with a 2D array to represent a game board
• How to generate random positions within the board’s boundaries using Random
• How to place game elements (snake and food) at their starting positions

🧠 Exercises — Practice Time!

💡 Beginner

• Write a program to check if a number is positive, negative, or zero using if.
• Print numbers from 1 to 10 using a for loop.
• Check if a number is divisible by 2 and 3 using &&.

🧪 Intermediate

• Use a switch to print weekday names from number (1-7).
• Use while loop to print first 5 even numbers.
• Use ternary operator to print “Even” or “Odd”.

🔥 Challenge

• Build a simple calculator that takes two numbers and an operator (+, -, *, /) and returns the result.
• Write a program to reverse digits of an integer using loop.
• Print a pattern using nested loops (like triangle or square of *).

⚠️ Common Mistakes in Operators & Control Flow (Java)

💡 Operators

• ❌ Confusing = (assignment) with == (equality)
• ❌ Using == for comparing strings instead of .equals()
• ❌ Forgetting parentheses in complex arithmetic expressions
• ❌ Dividing integers and expecting a float result (e.g., 5/2 gives 2)
• ❌ Misusing increment/decrement operators inside conditions
• ❌ Using ! without proper parentheses (e.g., !a == b)
• ❌ Writing a && b || c without understanding operator precedence
• ❌ Confusing & and && or | and ||
• ❌ Using a = b = c without realizing assignment returns a value
• ❌ Forgetting to cast during mixed-type arithmetic (e.g., int + double)

💡 if / else if / else

• ❌ Using if (condition); with an unintended semicolon
• ❌ Omitting { } for multiple statements in an if block
• ❌ Placing else without a preceding if
• ❌ Not covering all conditions in if / else if ladders
• ❌ Using assignment in conditions (if (a = 5) instead of ==)
• ❌ Confusing nested if statements without clear indentation
• ❌ Misunderstanding short-circuit logic in conditions (e.g., if (x != 0 && y/x > 1))
• ❌ Overcomplicating conditions instead of breaking them into steps
• ❌ Using if where a switch would be cleaner
• ❌ Forgetting the default else to handle unanticipated cases

💡 Switch-Case

• ❌ Omitting break in switch cases, causing fall-through
• ❌ Using non-constant expressions in case labels
• ❌ Forgetting the default case in switch statement
• ❌ Using incompatible types (e.g., float) in switch expressions
• ❌ Duplicating case values, causing compile errors
• ❌ Using == for string matching in switch instead of Java 7+ string switch
  • eg.

    // ❌ Incorrect way: using '==' for string comparison
    String fruit = "apple";
    if (fruit == "apple") {
        System.out.println("Fruit is apple");
    }
    
    // ✅ Better: using switch with String (Java 7+)
    switch (fruit) {
        case "apple":
            System.out.println("Fruit is apple");
            break;
        case "banana":
            System.out.println("Fruit is banana");
            break;
        default:
            System.out.println("Unknown fruit");
    }

• ❌ Using code after a break that is never reached
• ❌ Nesting switch statements without braces or clarity
• ❌ Including unreachable case blocks (e.g., duplicate constants)
• ❌ Not realizing switch supports only specific types (e.g., int, char, String)

💡 for loop

• ❌ Creating infinite loops with missing increment/decrement logic
• ❌ Off-by-one errors (e.g., using i <= n instead of <)
• ❌ Declaring loop variables outside unnecessarily
• ❌ Modifying loop counter inside the loop body incorrectly
• ❌ Nested loops using the same loop variable (e.g., i inside i)
• ❌ Using wrong data type for loop variable in large ranges
• ❌ Logic depends on side effects within loop headers
• ❌ Forgetting braces for nested loops
• ❌ Not initializing loop variable, leading to garbage values
• ❌ Confusing increment direction in for loops (e.g., i-- instead of i++)

💡 while loop

• ❌ Not updating loop control variable inside while (infinite loop)
• ❌ Checking an always-false condition causing loop to never run
• ❌ Using while instead of do-while when at least one execution is required
• ❌ Altering loop variables unintentionally inside the loop
• ❌ Not handling user input that causes while to repeat forever

💡 do-while loop

• ❌ Using do-while without understanding it runs at least once
• ❌ Adding semicolon after do incorrectly (do; { ... })
• ❌ Forgetting semicolon at the end of while(condition);
• ❌ Using break and continue poorly inside do-while
• ❌ Misplacing loop control variables outside of the loop body

✅ Operators & Control Flow (Java): Recap

• Use arithmetic operators (+, -, *, /, %) for calculations
• Use relational operators (==, !=, >, <, >=, <=) to compare values
• Logical operators (&&, ||, !) combine or invert conditions
• Use if, else if, else to handle conditional branching
• switch is useful for multi-way branching with fixed values
• for loops are ideal when the number of iterations is known
• while loops are used when the condition must be true before entering the loop
• do-while loops run the block at least once before checking the condition
• Ternary operator (condition ? trueValue : falseValue) offers a concise alternative to simple if-else
• Always initialize and update loop control variables to avoid infinite loops

🧠 Answers — With Explanations & Common Mistakes

💡 Beginner

• Check if a number is positive, negative, or zero

  public class PositiveNegativeZero {
      public static void main(String[] args) {
          int number = -5; // Change this to test different numbers
  
          if (number > 0) {
              System.out.println("Positive");
          } else if (number < 0) {
              System.out.println("Negative");
          } else {
              System.out.println("Zero");
          }
      }
  }

  ✅ Explanation:

  • if (number > 0) → Runs when the number is greater than zero.
  • else if (number < 0) → Runs when the number is less than zero.
  • else → Runs when the number is exactly zero.

  ❌ Common Mistakes:

  • Using = instead of == for equality (causes assignment, not comparison).
  • Using multiple if statements instead of else if, which may cause multiple outputs.
• Print numbers from 1 to 10 using a for loop

  public class PrintOneToTen {
      public static void main(String[] args) {
          for (int i = 1; i <= 10; i++) {
              System.out.println(i);
          }
      }
  }

  ✅ Explanation:

  • The loop starts at i = 1.
  • The condition i <= 10 ensures it runs until i reaches 10.
  • i++ increases i by 1 each time.

  ❌ Common Mistakes:

  • Using i < 10 instead of i <= 10, which prints only 1–9.
  • Resetting i inside the loop (e.g., i = 1;), causing an infinite loop.
  • Forgetting i++, which also causes an infinite loop.
• Check if a number is divisible by 2 and 3

  public class DivisibleByTwoAndThree {
      public static void main(String[] args) {
          int number = 12; // Change this to test
  
          if (number % 2 == 0 && number % 3 == 0) {
              System.out.println("Divisible by both 2 and 3");
          } else {
              System.out.println("Not divisible by both");
          }
      }
  }

  ✅ Explanation:

  • % (modulus) finds the remainder.
  • number % 2 == 0 → divisible by 2.
  • number % 3 == 0 → divisible by 3.
  • && means both conditions must be true.

  ❌ Common Mistakes:

  • Using || instead of && — checks if divisible by 2 or 3, not both.
  • Writing number % 2 = 0 — assignment instead of comparison, causing a compilation error.
  • Forgetting parentheses in complex conditions, leading to wrong results.

🧪 Intermediate

• Use a switch to print weekday names from number (1-7)

  public class WeekdaySwitch {
      public static void main(String[] args) {
          int day = 3; // Change to test different days (1-7)
  
          switch (day) {
              case 1: System.out.println("Monday"); break;
              case 2: System.out.println("Tuesday"); break;
              case 3: System.out.println("Wednesday"); break;
              case 4: System.out.println("Thursday"); break;
              case 5: System.out.println("Friday"); break;
              case 6: System.out.println("Saturday"); break;
              case 7: System.out.println("Sunday"); break;
              default: System.out.println("Invalid day");
          }
      }
  }

  ✅ Explanation: The switch statement checks the value of day against each case. When a match is found, the corresponding block runs. break stops the execution from continuing into the next case. The default case runs when none of the cases match.

  ❌ Common Mistakes:

  • Forgetting break, which causes the program to execute all following cases ("fall-through").
  • Not including a default case — this means invalid inputs won't be handled.
  • Using numbers outside the range 1–7 without realizing they'll go to default.
• Use while loop to print first 5 even numbers

  public class FirstFiveEvens {
      public static void main(String[] args) {
          int count = 0;  // How many even numbers printed
          int number = 2; // Start from the first even number
  
          while (count < 5) {
              System.out.println(number);
              number += 2; // Jump to the next even number
              count++;     // Keep track of how many printed
          }
      }
  }

  ✅ Explanation: The while loop runs as long as count < 5. Each time it prints the current even number, increases it by 2, and increments count to eventually stop the loop.

  ❌ Common Mistakes:

  • Forgetting to increment count or number — causes an infinite loop.
  • Starting number at 0 if you want numbers from 2 — leads to wrong sequence.
  • Using <= 5 instead of < 5 with count — prints 6 numbers instead of 5.
• Use ternary operator to print “Even” or “Odd”

  public class EvenOddTernary {
      public static void main(String[] args) {
          int num = 7; // Change this to test
  
          String result = (num % 2 == 0) ? "Even" : "Odd";
          System.out.println(result);
      }
  }

  ✅ Explanation: The ternary operator ?: is a shorthand for if-else. Here, (num % 2 == 0) checks if the number is divisible by 2. If true, it assigns "Even" to result, otherwise "Odd".

  ❌ Common Mistakes:

  • Forgetting parentheses around the condition, making it harder to read or leading to precedence issues.
  • Using = instead of == in the condition — causes unintended assignment.
  • Placing a semicolon ; inside the ternary — breaks the expression.

🔥 Challenge

• Build a simple calculator (fixed values)

  public class SimpleCalculator {
      public static void main(String[] args) {
          int a = 10, b = 5;
          char op = '*';
  
          switch (op) {
              case '+': System.out.println(a + b); break;
              case '-': System.out.println(a - b); break;
              case '*': System.out.println(a * b); break;
              case '/':
                  if (b != 0) {
                      System.out.println(a / b);
                  } else {
                      System.out.println("Cannot divide by zero");
                  }
                  break;
              default: System.out.println("Invalid operator");
          }
      }
  }

  ✅ Explanation: The switch checks the value of op and performs the corresponding arithmetic operation. For division, we check b != 0 to prevent division by zero.

  ❌ Common Mistakes:

  • Forgetting break — leads to executing multiple cases unintentionally.
  • Using "+" (String) instead of '+' (char) for op.
  • Not checking for division by zero — causes runtime exception.
• Reverse digits of an integer

  public class ReverseNumber {
      public static void main(String[] args) {
          int num = 1234;
          int reversed = 0;
  
          while (num != 0) {
              int digit = num % 10;
              reversed = reversed * 10 + digit;
              num /= 10;
          }
  
          System.out.println("Reversed: " + reversed);
      }
  }

  ✅ Explanation: We extract the last digit with num % 10, multiply the reversed number by 10 to shift digits, then add the extracted digit. We remove the last digit from num using integer division.

  ❌ Common Mistakes:

  • Forgetting num /= 10 — causes an infinite loop.
  • Mixing up % and / — leads to incorrect results.
  • Not handling negative numbers if required.
• Print patterns using nested loops (triangle & square)

  public class TrianglePattern {
      public static void main(String[] args) {
          int rows = 5;
  
          // Triangle pattern
          for (int i = 1; i <= rows; i++) {
              for (int j = 1; j <= i; j++) {
                  System.out.print("* ");
              }
              System.out.println();
          }
      }
  }

  public class SquarePattern {
      public static void main(String[] args) {
          int size = 5;
  
          // Square pattern
          for (int i = 1; i <= size; i++) {
              for (int j = 1; j <= size; j++) {
                  System.out.print("* ");
              }
              System.out.println();
          }
      }
  }

  ✅ Explanation:

  • Triangle: Outer loop controls the number of rows, inner loop prints stars equal to the current row number.
  • Square: Both outer and inner loops run size times, so each row has the same number of stars.
  • System.out.print keeps printing on the same line, while System.out.println() moves to the next line after each row.

  ❌ Common Mistakes:

  • Swapping print and println — breaks the pattern layout.
  • Using < instead of <= in loops — causes missing stars or rows.
  • Accidentally reusing the same loop variable in both loops — leads to incorrect output.
• Calculator with user input (handles invalid input)

  import java.util.Scanner;
  
  public class CalculatorUserInput {
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
  
          try {
              System.out.print("Enter first number: ");
              if (!sc.hasNextDouble()) {
                  System.out.println("Invalid number! Please restart the program.");
                  return;
              }
              double a = sc.nextDouble();
  
              System.out.print("Enter operator (+, -, *, /): ");
              String opInput = sc.next();
              if (opInput.length() != 1 || "+-*/".indexOf(opInput.charAt(0)) == -1) {
                  System.out.println("Invalid operator! Please restart the program.");
                  return;
              }
              char op = opInput.charAt(0);
  
              System.out.print("Enter second number: ");
              if (!sc.hasNextDouble()) {
                  System.out.println("Invalid number! Please restart the program.");
                  return;
              }
              double b = sc.nextDouble();
  
              switch (op) {
                  case '+': System.out.println("Result: " + (a + b)); break;
                  case '-': System.out.println("Result: " + (a - b)); break;
                  case '*': System.out.println("Result: " + (a * b)); break;
                  case '/':
                      if (b != 0) {
                          System.out.println("Result: " + (a / b));
                      } else {
                          System.out.println("Cannot divide by zero");
                      }
                      break;
              }
          } finally {
              sc.close();
          }
      }
  }

  ✅ Explanation: This version checks for invalid numbers using hasNextDouble() and validates the operator string before performing the calculation.

  ❌ Common Mistakes:

  • Not checking hasNextDouble() before reading numbers — causes program crash if letters are entered.
  • Accepting multi-character operators like ++ — this code prevents that.
  • Skipping finally or close() — leads to resource leaks.
• Complex calculator with multiple operations (handles invalid input)

  import java.util.Scanner;
  
  public class ComplexCalculator {
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          boolean running = true;
  
          while (running) {
              try {
                  System.out.print("Enter first number: ");
                  if (!sc.hasNextDouble()) {
                      System.out.println("Invalid number! Please try again.");
                      sc.next(); // clear wrong input
                      continue;
                  }
                  double a = sc.nextDouble();
  
                  System.out.print("Enter operator (+, -, *, /, %): ");
                  String opInput = sc.next();
                  if (opInput.length() != 1 || "+-*/%".indexOf(opInput.charAt(0)) == -1) {
                      System.out.println("Invalid operator! Please try again.");
                      continue;
                  }
                  char op = opInput.charAt(0);
  
                  System.out.print("Enter second number: ");
                  if (!sc.hasNextDouble()) {
                      System.out.println("Invalid number! Please try again.");
                      sc.next(); // clear wrong input
                      continue;
                  }
                  double b = sc.nextDouble();
  
                  switch (op) {
                      case '+': System.out.println("Result: " + (a + b)); break;
                      case '-': System.out.println("Result: " + (a - b)); break;
                      case '*': System.out.println("Result: " + (a * b)); break;
                      case '/':
                          if (b != 0) {
                              System.out.println("Result: " + (a / b));
                          } else {
                              System.out.println("Cannot divide by zero");
                          }
                          break;
                      case '%':
                          if (b != 0) {
                              System.out.println("Result: " + (a % b));
                          } else {
                              System.out.println("Cannot perform modulo by zero");
                          }
                          break;
                  }
  
                  System.out.print("Do you want to calculate again? (yes/no): ");
                  String choice = sc.next();
                  if (!choice.equalsIgnoreCase("yes")) {
                      running = false;
                  }
  
              } catch (Exception e) {
                  System.out.println("Unexpected error: " + e.getMessage());
                  sc.next(); // clear bad input
              }
          }
  
          sc.close();
          System.out.println("Calculator closed.");
      }
  }

  ✅ Explanation: This version uses hasNextDouble() and operator validation to prevent crashes. It runs in a loop and keeps asking until the user chooses to exit.

  ❌ Common Mistakes:

  • Not clearing invalid input with sc.next() — without this, the loop keeps reading the same bad input forever.
  • Using == for strings instead of equalsIgnoreCase() — this breaks string comparison.
  • Forgetting to validate division and modulus by zero.

🧪 Modulus with Smaller Dividend

• Example:

  public class ModulusDemo {
      public static void main(String[] args) {
          System.out.println(2 % 3);       // integer modulus
          System.out.println(2 % 3.0);     // double modulus
          double result = 5.5 % 2.2;       // raw double result
          System.out.println(Math.round(result * 100.0) / 100.0); // rounded to 2 decimals
      }
  }
  

• Output:

  2
  2.0
  1.1
  

• ✅ Explanation: The modulus operator (%) returns the remainder after division.

  • 2 % 3 → Since 2 is smaller than 3, the quotient is 0 and the remainder is 2 (integer).
  • 2 % 3.0 → One operand is a double, so Java promotes the result to double, giving 2.0.
  • 5.5 % 2.2 → The quotient is 2 (truncated), raw result is 1.0999999999999996 due to floating-point precision. Using rounding: Math.round(result * 100.0) / 100.0 → 1.1.
  Formula: remainder = dividend - (divisor × truncatedQuotient)
  • Dividend: 2
  • Divisor: 3
  • Integer division: 2 / 3 = 0 (since 2 is smaller than 3, the quotient is zero)
  • Remainder: 2 - (3 × 0) = 2

• ❌ Common Mistakes:

  • Thinking modulus only works for integers — it also works with floating-point numbers.
  • Confusing modulus with division — modulus gives the remainder, not the quotient.
  • Forgetting type promotion — when one operand is double, the result is also double.
  • Not accounting for floating-point precision — use rounding for cleaner results.

🎉 That's it! You've now solved all practice questions from basic checks to loops and logic!

Now you can create decisions, loops, and intelligent logic inside your Java code. This is the step where code becomes powerful!

— Blog by Aelify (ML2AI.com)

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