Java Syntax and Structure with Examples

1. Java Program Structure (Classes and Methods)

Understanding Java Classes
- A class is a blueprint for creating objects in Java. Here’s a simple example of a Java class:
```
java

public class MyFirstClass {
    // This is a member variable
    int number = 5;

    // This is a method
    public void displayNumber() {
        System.out.println("Number: " + number);
    }
}
```
- In this example, MyFirstClass is a Java class, number is a variable, and displayNumber is a method that prints the value of number.
The Main Method
- The main method is the entry point for any Java application. Here’s a simple "Hello, World!" program:
```
java

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}
```
- In this code, public static void main(String[] args) is the main method, where execution starts. System.out.println("Hello, World!"); prints text to the console.

Creating and Calling Methods

Methods allow us to break down code into smaller, reusable parts. Here’s an example with method declaration and invocation:

java
public class MathOperations {
    // Method to add two numbers
    public int add(int a, int b) {
        return a + b;
    }

    public static void main(String[] args) {
        MathOperations math = new MathOperations();
        int result = math.add(5, 3);
        System.out.println("Sum: " + result);
    }
}

Here, add is a method that takes two parameters (int a and int b), adds them, and returns the sum. The main method calls add, and the result is printed.

2. Comments in Java

Single-line Comments (//)

java

// This is a single-line comment explaining the line below
System.out.println("Hello, World!");

Multi-line Comments (/* ... */)

java

/*
 * This is a multi-line comment.
 * It can span multiple lines.
 */
System.out.println("Java comments are helpful!");

Documentation Comments (/** ... */)

java

/**
 * This method returns the sum of two numbers.
 * @param a The first number
 * @param b The second number
 * @return The sum of a and b
 */
public int add(int a, int b) {
    return a + b;
}

Documentation comments like these are used for creating JavaDocs.

3. Java Statements

Example of Variable Declaration and Assignment Statements

java

int number = 10;  // Declaring and initializing a variable
String message = "Java is fun!";
System.out.println(message);

Syntax Rules
- Every statement ends with a semicolon (;). Code blocks are wrapped in braces ({}). For example:
```
java

if (number > 5) {
    System.out.println("Number is greater than 5");
}
```

4. Java Keywords

Common Keywords Example

java

public class Example {
    public static void main(String[] args) {
        int number = 10;  // 'int' is a keyword for integer type
        final int CONSTANT = 100;  // 'final' makes CONSTANT unchangeable
        System.out.println(number + CONSTANT);
    }
}

Here, public, class, static, void, int, and final are keywords.

Reserved Words in Java

Words like new, if, else, switch, etc., are reserved in Java and cannot be used as variable names. Here’s an example using if and else:

java

public class CheckEvenOdd {
    public static void main(String[] args) {
        int number = 4;

        if (number % 2 == 0) {
            System.out.println("Number is even");
        } else {
            System.out.println("Number is odd");
        }
    }
}

Cse Wallah

Java Syntax and Structure with Examples