Exceptions: a custom unchecked exception skips the throws clause entirely

Extending RuntimeException makes a custom exception unchecked: it can be thrown from any method without a throws declaration and callers are free to ignore it, which suits programmer errors rather than recoverable conditions.

Code

class NegativeQuantityException extends RuntimeException {
    NegativeQuantityException(int quantity) {
        super("quantity cannot be negative: " + quantity);
    }
}
static int order(int quantity) {
    if (quantity < 0) {
        throw new NegativeQuantityException(quantity);
    }
    return quantity;
}
try {
    order(-3);
} catch (NegativeQuantityException e) {
    System.out.println(e.getClass().getSimpleName() + ": " + e.getMessage());
}

Output

NegativeQuantityException: quantity cannot be negative: -3