Custom Exceptions

1. What is a Custom Exception?

A custom exception is an exception class you create yourself, usually to represent a very specific problem related to your own application's logic, which Java's built-in exceptions don't already describe well.

2. Why is it used?

Built-in exceptions are quite general. Creating your own, clearly named exception (like InsufficientBalanceException) makes your error handling more meaningful and easier for other developers to understand.

3. Real-Life Example

Think of a company creating its own specific complaint category, like "Late Delivery Complaint," instead of just using a generic "Issue" label for every problem. A custom exception gives a specific, meaningful name to a specific kind of problem.

4. Syntax

java
class CustomExceptionName extends Exception {
    CustomExceptionName(String message) {
        super(message);
    }
}

5. Example Program

java
class InsufficientBalanceException extends Exception {
    InsufficientBalanceException(String message) {
        super(message);
    }
}

public class CustomExceptionDemo {
    static void withdraw(double balance, double amount) throws InsufficientBalanceException {
        if (amount > balance) {
            throw new InsufficientBalanceException("Not enough balance!");
        }
        System.out.println("Withdrawal successful.");
    }

    public static void main(String[] args) {
        try {
            withdraw(1000, 5000);
        } catch (InsufficientBalanceException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

Output:

Error: Not enough balance!

6. Key Points to Remember

• A custom exception usually extends Exception (checked) or RuntimeException (unchecked), depending on the intended use.
• Using super(message) passes the error message up to the built-in exception behaviour.
• Custom exceptions make error messages in large applications much clearer and more specific.