Custom Exceptions

❔ What is a Custom Exception?

A custom exception is a user-defined class that extends either:

• Exception → for checked exceptions

• RuntimeException → for unchecked exceptions

You create them when built-in exceptions are not descriptive or specific enough for your program’s needs.

💡 Real-Life Analogy

Imagine you're building a banking system.

Java doesn’t have InsufficientFundsException, so you create your own to make error handling clear and meaningful.

❗Creating a Custom Exception

1️⃣ Step 1: Define the Exception

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

✅ This defines a checked exception because it extends Exception.

2️⃣ Step 2: Use the Exception with throw

public class Main {
    static void validateAge(int age) throws InvalidAgeException {
        if (age < 18) {
            throw new InvalidAgeException("Age must be 18 or above");
        } else {
            System.out.println("Eligible to vote");
        }
    }

    public static void main(String[] args) {
        try {
            validateAge(16);
        } catch (InvalidAgeException e) {
            System.out.println("Caught custom exception: " + e.getMessage());
        }
    }
}

Caught custom exception: Age must be 18 or above

❗Custom Unchecked Exception (Optional)

If you don’t want Java to force you to handle the exception (like runtime ones), extend RuntimeException:

class TooManyAttemptsException extends RuntimeException {
    public TooManyAttemptsException(String msg) {
        super(msg);
    }
}

**Use just like above, but no need to declare with throws.

❔When to Use Custom Exceptions?

• When built-in exceptions don’t convey the meaning

• To make your code more readable and maintainable

• For domain-specific errors (e.g., LowBalanceException, UserNotFoundException)

🧾 Recap Table