0. Comments

// Single-line comment
/* Multi-line comment */
/** Documentation comment for classes and methods */

1. Basic Output

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

2. Reading Input from Console

import java.util.Scanner;

public class ReadInput {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double value = scanner.nextDouble();
        System.out.println("You entered: " + value);
    }
}

3. Identifiers

Identifiers in Java are sequences composed of letters, digits, underscores (_), and dollar signs ($). Rules include:

• Must start with a letter, underscore, or dollar sign—not a digit.
• Cannot be Java keywords (e.g., class, public).
• Cannot be the literals true, false, or null.
• Can be of any length.

Java is case-sensitive. Prefer descriptive names over abbreviations. Avoid using single characters except in auto-generated code.

4. Variable Declaration

// Declaration only
int count;
double price;
long total, balance;

// Declaration with initialization
int score = 100;
double width = 5.5, height = 3.2;

5. Assignment Statements

int y = 1;
double radius = 1.0;
int result = 5 * (3 / 2);  // Integer division: 3/2 = 1
result = y + 1;
result = result + 1;       // Increment by 1

6. Constants

final double PI = 3.14159;

7. Naming Conventions

Use clear, descriptive names for variables, constants, classes, and methods:

• Variables and methods: Use camelCase—start with lowercase, capitalize subsequent words (e.g., studentCount, calculateArea).
• Classes: Use PascalCase—capitalize the first lettter of every word (e.g., CircleCalculator, FileHandler).
• Constants: Use uppercase with underscores between words (e.g., MAX_SIZE, DEFAULT_TIMEOUT).