Exceptions: a custom checked exception forces every caller to handle it

Extending Exception (not RuntimeException) makes a custom exception checked, so any method that throws it must declare it and any caller must either catch it or propagate it, enforced entirely at compile time.

Code

class InsufficientFundsException extends Exception {
    InsufficientFundsException(String message) {
        super(message);
    }
}
class Account {
    static void withdraw(int balance, int amount) throws InsufficientFundsException {
        if (amount > balance) {
            throw new InsufficientFundsException("need " + amount + " but have " + balance);
        }
    }
}
try {
    Account.withdraw(50, 100);
} catch (InsufficientFundsException e) {
    System.out.println(e.getClass().getSimpleName() + ": " + e.getMessage());
}

Output

InsufficientFundsException: need 100 but have 50