📘 [3] Java Syntax & Basics — Structure, Statements, and Starting Right!

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Java 📘 [3] Java Syntax & Basics — Structure, Statements, and Starting Right!
📘 [3] Java Syntax & Basics — Structure, Statements, and Starting Right!

📘 [3] Java Syntax & Basics — Structure, Statements, and Starting Right!

🧱 Java: A Structured Language

Java is known for being verbose — but also clear and strict. Its syntax borrows from C/C++, but adds safety, structure, and readability. To code effectively in Java, you must understand how files, classes, methods, statements, and variables come together to form a complete program.

📄 The Minimal Java Program

This is the smallest valid Java program you can write:

public class Hello {
  public static void main(String[] args) {
    System.out.println("Hello, Java!");
  }
}
  • public class Hello → The class must match the filename (Hello.java)
  • main → The entry point for any Java program
  • System.out.println → Built-in print function (prints to console)

⚙️ Anatomy of a Java Program

  • Every Java file contains at least one class
  • main() is where the program starts execution
  • Curly braces { } group code into blocks
  • Every statement ends in a semicolon ;
  • // is a single-line comment, /* */ for multi-line

🗂️ File Structure & Naming Rules

  • The filename must match the public class name
  • Class names start with an uppercase letter (e.g., MyClass)
  • Method names start with lowercase (e.g., main)
  • Java is case-sensitive → Main ≠ main

🔤 Variables & Data Types

Java is statically typed, so every variable must have a type. Examples:

int age = 25;
double pi = 3.14;
char grade = 'A';
boolean isJavaFun = true;
String name = "Aelify";

📋 Common Data Types

  • int → Whole numbers (e.g., 42)
  • double → Decimal numbers (e.g., 3.14)
  • char → Single characters (e.g., 'A')
  • boolean → true / false
  • String → Text (actually a class, not a primitive)

✏️ Comments & Readability

// This is a single-line comment

/*
 This is a 
 multi-line comment
*/
  • Use comments to explain why, not what
  • Good indentation makes code readable
  • Use meaningful variable names — avoid int x, prefer int itemCount

🚧 Java Compilation Flow

  1. You write code in Hello.java
  2. Compile with: javac Hello.java → creates Hello.class
  3. Run with: java Hello (no .class or .java)

Example output:

Hello, Java!

🧠 Pro Tips for Beginners

  • 👁️ Always match class name with filename
  • 💡 Use an IDE — helps catch syntax errors early
  • 🧪 Practice writing, compiling, and running without an IDE too
  • 🧼 Format your code! Use auto-format (Ctrl+Alt+L in IntelliJ)
  • 🔎 Understand errors — read the compiler messages carefully

🧪 Try This Exercise

public class PersonalInfo {
  public static void main(String[] args) {
    String name = "Aelify";
    int age = 21;
    boolean learningJava = true;

    System.out.println("Name: " + name);
    System.out.println("Age: " + age);
    System.out.println("Learning Java? " + learningJava);
  }
}

💡 Try modifying the values and adding a new variable for height!

✅ Recap

Java is strict, but predictable. Once you understand its structure — classes, main method, semicolons, and variable types — everything starts to click. You’ve now written your first real Java code and understood how it flows from source file to output.

✍️ Java Syntax Basics

✅ Semicolons

Every statement ends with a semicolon ;.

✅ Case Sensitivity

Java is case-sensitive: System ≠ system

✅ Braces { } define blocks

if (true) {
  System.out.println("Inside block");
}

🧮 Variables & Data Types

Type Description Example
int Whole numbers int age = 25;
double Decimal numbers double pi = 3.14;
char Single character char grade = 'A';
boolean True/False boolean alive = true;
String Text (non-primitive) String name = "Aelify";

🧪 Example:

int age = 30;
double height = 5.9;
boolean isStudent = true;
String language = "Java";

➕ Operators

Operator Purpose Example
+ - * / % Arithmetic a + b, x % y
== != > < >= <= Comparison a == b
&& || ! Logical a > 0 && b < 5
= Assignment x = 5

🧪 Example:

int a = 10, b = 5;
System.out.println("Sum: " + (a + b));
System.out.println("Is A greater? " + (a > b));

🔁 Conditional Statements

✅ If-Else

int score = 85;

if (score >= 90) {
  System.out.println("Grade A");
} else if (score >= 75) {
  System.out.println("Grade B");
} else {
  System.out.println("Grade C");
}

✅ Switch Case

char grade = 'B';

switch (grade) {
  case 'A':
    System.out.println("Excellent");
    break;
  case 'B':
    System.out.println("Good Job");
    break;
  default:
    System.out.println("Keep Trying");
}

🔁 Loops in Java

✅ For Loop

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

✅ While Loop

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

✅ Do-While Loop

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

📚 User Input (Scanner)

import java.util.Scanner;

public class InputExample {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter your name: ");
    String name = sc.nextLine();
    System.out.println("Hello, " + name + "!");
  }
}

🧠 Exercises — Practice Zone with Answers

💡 Beginner

  • ✅ Write a Java program that adds two integers.
import java.util.Scanner;

public class AddTwoNumbers {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter first number: ");
    int a = sc.nextInt();
    System.out.print("Enter second number: ");
    int b = sc.nextInt();
    int sum = a + b;
    System.out.println("Sum = " + sum);
  }
}
  • ✅ Print multiplication table of 7 using a loop.
public class TableOf7 {
  public static void main(String[] args) {
    for (int i = 1; i <= 10; i++) {
      System.out.println("7 x " + i + " = " + (7 * i));
    }
  }
}
  • ✅ Check if a number is even or odd.
import java.util.Scanner;

public class EvenOdd {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter a number: ");
    int num = sc.nextInt();
    if (num % 2 == 0)
      System.out.println(num + " is Even");
    else
      System.out.println(num + " is Odd");
  }
}

🧪 Intermediate

  • ✅ Accept user age and classify: “Child”, “Teen”, “Adult”, “Senior”.
import java.util.Scanner;

public class AgeClassifier {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter your age: ");
    int age = sc.nextInt();

    if (age < 13)
      System.out.println("Child");
    else if (age < 20)
      System.out.println("Teen");
    else if (age < 60)
      System.out.println("Adult");
    else
      System.out.println("Senior");
  }
}
  • ✅ Calculate factorial of a number using for loop.
import java.util.Scanner;

public class Factorial {
  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 = " + fact);
  }
}
  • ✅ Take a sentence input and print each word on a new line.
import java.util.Scanner;

public class SplitSentence {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter a sentence: ");
    String sentence = sc.nextLine();

    String[] words = sentence.split(" ");
    for (String word : words) {
      System.out.println(word);
    }
  }
}

🔥 Challenge

  • ✅ FizzBuzz (1 to 100): Print “Fizz” if divisible by 3, “Buzz” if divisible by 5, “FizzBuzz” if both, else the number.
public class FizzBuzz {
  public static void main(String[] args) {
    for (int i = 1; i <= 100; i++) {
      if (i % 3 == 0 && i % 5 == 0)
        System.out.println("FizzBuzz");
      else if (i % 3 == 0)
        System.out.println("Fizz");
      else if (i % 5 == 0)
        System.out.println("Buzz");
      else
        System.out.println(i);
    }
  }
}

✅ Recap

You’ve covered the most essential building blocks of Java:

  • Syntax and formatting rules
  • Variables and data types
  • Conditions and logic
  • Looping structures
  • User input with Scanner
  • Tons of hands-on code and exercises — with answers!

⚠️ Common Mistakes

  • ❌ Missing semicolons (;) — Every Java statement must end with a semicolon.
  • ❌ Incorrect capitalization — Java is case-sensitive. System ≠ system, Main ≠ main.
  • ❌ Using assignment (=) instead of comparison (==) in conditions.
  • ❌ Unmatched or missing braces { } — leads to "illegal start of type" or logical errors.
  • ❌ Missing class or main method — program won’t run without: public static void main(String[] args)
  • ❌ Writing code outside class — all code must be inside a class block.
  • ❌ Incorrect file name — must match the public class name exactly (e.g., HelloWorld.java for public class HelloWorld).
  • ❌ Using undeclared variables — must declare before use.
  • ❌ Type mismatch — assigning a String to int or double to int without casting.
  • ❌ Uninitialized variables — Java won’t let you use a local variable before assigning a value.
  • ❌ Incorrect char declaration — use single quotes: char c = 'a'; (not "a").
  • ❌ Not importing java.util.Scanner before using it.
  • ❌ Using nextLine() after nextInt() or nextDouble() without consuming the newline.
  • ❌ Infinite loops — forgetting to update loop counters (like i++ in while/for).
  • ❌ Wrong loop condition — such as i >= 0 with i++, causing endless loop.
  • ❌ Confusing = with == in if conditions.
  • ❌ Using else if without a preceding if.
  • ❌ Switch-case without break; — leads to fall-through bugs.
  • ❌ Concatenating numbers without parentheses — "Sum: " + a + b gives string like "Sum: 105" instead of 15.
  • ❌ Trying to run Java file without compiling it first — must run javac FileName.java before java FileName.

🛠️ Example Fix for Scanner Bug

If you use nextInt() followed by nextLine(), always clear the newline buffer:

import java.util.Scanner;

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

    System.out.print("Enter your age: ");
    int age = sc.nextInt();
    sc.nextLine(); // ⚠️ This clears the \n left behind by nextInt()

    System.out.print("Enter your name: ");
    String name = sc.nextLine();

    System.out.println("Hi " + name + ", you are " + age + " years old.");
  }
}

You're now ready to build interactive, logical Java programs. Up next, we’ll explore variables in detail — primitive vs reference, memory, type casting, and more!

In our next post, we’ll dive into 🧮 [4] Data Types & Variables — The Core Foundation of Java!

— Blog by Aelify (ML2AI.com)

Location: India