Exception Handling: Managing the Unexpected

In Java, an Exception is an event that disrupts the normal flow of a program (like trying to divide by zero or opening a file that doesn't exist). Exception Handling is the process of "catching" these errors so your program doesn't crash.

1. The Try-Catch Block

This is the most common way to handle exceptions. You "try" a block of code, and if it fails, the "catch" block executes.

try {
    int result = 10 / 0; // This will throw an ArithmeticException
} catch (ArithmeticException e) {
    System.out.println("Error: You cannot divide by zero!");
}

2. Keywords to Know

3. Checked vs. Unchecked Exceptions

• Unchecked (Runtime): Errors caused by bad logic (e.g., NullPointerException, ArrayIndexOutOfBounds). You aren't forced to catch these, but you should fix the logic.
• Checked: Errors outside the program's control (e.g., FileNotFoundException). Java forces you to either catch these or declare them with throws.

💻 Full Practical Example: The Safe Input

Copy this code to see how to prevent a program from crashing when a user enters bad data:

import java.util.Scanner;
import java.util.InputMismatchException;

public class SafeDivider {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        try {
            System.out.print("Enter a number: ");
            int num = scanner.nextInt();
            System.out.println("Result: " + (100 / num));
        } 
        catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero.");
        } 
        catch (InputMismatchException e) {
            System.out.println("That wasn't a valid integer!");
        } 
        finally {
            System.out.println("Closing scanner...");
            scanner.close();
        }
        
        System.out.println("Program continues safely...");
    }
}

4. Throwing Your Own Exceptions

You can use the throw keyword to create custom rules for your program.

public class Main {

    public static void checkAge(int age) {
        if (age < 18) {
            throw new SecurityException("Access Denied: You are too young.");
        } else {
            System.out.println("Access Granted.");
        }
    }

    public static void main(String[] args) {

        try {
            checkAge(15); // change value to test
        } catch (SecurityException e) {
            System.out.println("Exception caught: " + e.getMessage());
        }

        System.out.println("Program continues...");
    }
}

💡 Challenge: The Array Guardian

1. Create an array of 3 integers.
2. Ask the user for an index number.
3. Use a try-catch block to access that index.
4. Catch the ArrayIndexOutOfBoundsException and print a friendly message like "That index doesn't exist!".
5. Use a finally block to print "Search complete."