Java Exception Handling

1. Try and Catch

The try block encloses code that may throw an exception. The catch block handles it.

public class TryCatchDemo {
    public static void main(String[] args) {
        try {
            int[] arr = {10, 20, 30};
            System.out.println(arr[5]); // Causes ArrayIndexOutOfBoundsException
        } catch (Exception e) {
            System.out.println("Error occurred: " + e.getMessage());
        }
    }
}

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2. Finally Block

The finally block executes whether an exception occurs or not. Useful for closing files, database connections, or releasing resources.

public class FinallyDemo {
    public static void main(String[] args) {
        try {
            System.out.println("Inside try block");
            int divide = 10 / 0; // Throws error
        } catch (Exception e) {
            System.out.println("Caught: " + e.getMessage());
        } finally {
            System.out.println("Finally always executes");
        }
    }
}

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3. The throw Keyword

The throw keyword is used to manually throw an exception.

public class ThrowDemo {

    static void checkAge(int age) {
        if (age < 18) {
            throw new IllegalArgumentException("Access denied – You must be at least 18.");
        } else {
            System.out.println("Access granted");
        }
    }

    public static void main(String[] args) {
        checkAge(15);
    }
}

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4. The throws Keyword

The throws keyword declares that a method may throw an exception. The calling method must handle it.

import java.io.IOException;

public class ThrowsDemo {

    static void readData() throws IOException {
        throw new IOException("Unable to read data");
    }

    public static void main(String[] args) {
        try {
            readData();
        } catch (IOException e) {
            System.out.println("Handled IOException: " + e.getMessage());
        }
    }
}

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5. Checked vs Unchecked Exceptions

Checked Exceptions: Occur at compile-time (e.g., IOException, SQLException).

Unchecked Exceptions: Occur at runtime (e.g., NullPointerException, ArithmeticException).

6. Creating a Custom Exception

class InvalidAgeException extends Exception {
    public InvalidAgeException(String message) {
        super(message);
    }
}

public class CustomExceptionDemo {

    static void validate(int age) throws InvalidAgeException {
        if (age < 18) {
            throw new InvalidAgeException("Custom Error: Age must be 18 or above");
        }
        System.out.println("Valid Age!");
    }

    public static void main(String[] args) {
        try {
            validate(15);
        } catch (InvalidAgeException e) {
            System.out.println(e.getMessage());
        }
    }
}

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