Custom Exceptions

A custom exception is a user-defined class that extends the Exception or Runtime.
It’s used to signal specific, meaningful errors in your application.

Example

Checked Exception

class class AgeTooLowException extends Exception {
    public AgeTooLowException(String message) {
        super(message);
    }
}
 
void validateAge(int age) throws AgeTooLowException {
    if (age < 18) {
        throw new AgeTooLowException("Age must be 18 or above.");
    }
}

Unchecked Exception

class InvalidUsernameException extends RuntimeException {
    public InvalidUsernameException(String message) {
        super(message);
    }
}

When to Use Custom Exceptions

• To represent domain-specific errors.
• To provide clearer error messages.
• To simplify debugging and logging.
• To enforce business rules (e.g. InsufficientBalanceException).

Best Practices

• Inherit from Exception or RuntimeException.
• Include constructors with message + cause.
• Name them clearly: end with Exception.

Real-World Analogy

Java’s build-in exceptions = road signs.
Custom exceptions = your own business-specific warnings.

Tip

Be ready to explain:

• why and when you’d use a custom exception,
• checked and unchecked choice,
• how custom exceptions improve code readability and maintainability.

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Parent: _Exceptions