public class Functionality {

	public static int add(int a, int b) throws NegativeNumberException {

		if (a < 0 || b < 0) {
			throw new NegativeNumberException();
		}

		int ergebnis = a + b;

		return ergebnis;
	}
}

=======================================================================================================================

public class NegativeNumberException extends Exception {

	/**
	 *
	 */
	private static final long serialVersionUID = 1L;

}