Lesson 15: Custom Exceptions & Checked vs Unchecked

Checked exceptions force callers to handle or declare them (compile-time enforced); unchecked ones (RuntimeException) don't — use each deliberately.

Real-time example

A banking API defines InsufficientFundsException (checked) so every caller is forced to explicitly handle a declined withdrawal.

class InsufficientFundsException extends Exception {
    InsufficientFundsException(String msg) { super(msg); }
}
void withdraw(double amt, double balance) throws InsufficientFundsException {
    if (amt > balance)
        throw new InsufficientFundsException("Balance too low for Rs " + amt);
}

What's happening

Because it extends Exception (not RuntimeException), any caller of withdraw must either catch it or declare throws themselves.

Reserve checked exceptions for recoverable business conditions; use unchecked for programmer errors (bad args, null).