Java Exception Handling

// Without exception handling — program crashes
int result = 10 / 0;  // ArithmeticException!

// With exception handling — program continues
try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero!");
}

try {
    // Code that might throw an exception
    int[] arr = {1, 2, 3};
    System.out.println(arr[5]);
} catch (ArrayIndexOutOfBoundsException e) {
    // Handle the exception
    System.out.println("Invalid index: " + e.getMessage());
} finally {
    // Always executes (cleanup code)
    System.out.println("This always runs.");
}

Throwable
├── Error (serious — don't catch)
│   ├── OutOfMemoryError
│   └── StackOverflowError
└── Exception
    ├── IOException (checked)
    ├── SQLException (checked)
    └── RuntimeException (unchecked)
        ├── NullPointerException
        ├── ArithmeticException
        ├── ArrayIndexOutOfBoundsException
        └── NumberFormatException

try {
    String str = null;
    System.out.println(str.length());
} catch (NullPointerException e) {
    System.out.println("Null reference: " + e.getMessage());
} catch (Exception e) {
    System.out.println("General error: " + e.getMessage());
}

// throw — manually throw an exception
public static void validateAge(int age) {
    if (age < 0) {
        throw new IllegalArgumentException("Age cannot be negative: " + age);
    }
}

// throws — declare that a method might throw
public static void readFile(String path) throws IOException {
    FileReader reader = new FileReader(path);
    // ...
}

public class InsufficientBalanceException extends Exception {
    private double balance;
    private double amount;

    public InsufficientBalanceException(double balance, double amount) {
        super(String.format("Cannot withdraw ₹%.2f. Balance: ₹%.2f", amount, balance));
        this.balance = balance;
        this.amount = amount;
    }
}

try (Scanner scanner = new Scanner(new File("data.txt"))) {
    while (scanner.hasNextLine()) {
        System.out.println(scanner.nextLine());
    }
} catch (FileNotFoundException e) {
    System.out.println("File not found: " + e.getMessage());
}
// Scanner is automatically closed here

import java.util.Scanner;

public class SafeDivision {
    public static double divide(int a, int b) throws ArithmeticException {
        if (b == 0) throw new ArithmeticException("Division by zero");
        return (double) a / b;
    }

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

        try {
            System.out.print("Enter numerator: ");
            int a = Integer.parseInt(scanner.nextLine());

            System.out.print("Enter denominator: ");
            int b = Integer.parseInt(scanner.nextLine());

            double result = divide(a, b);
            System.out.printf("Result: %.2f%n", result);
        } catch (NumberFormatException e) {
            System.out.println("Please enter valid numbers.");
        } catch (ArithmeticException e) {
            System.out.println("Error: " + e.getMessage());
        } finally {
            scanner.close();
            System.out.println("Calculator closed.");
        }
    }
}