### Question:
ID implements Serializable { public static boolean isDefined(ID id) { return id != null && id.isSet(); }  static ID of(int value); @Override String toString(); int get(); boolean isSet(); static boolean isDefined(ID id); Optional<T> ifSet(Function<Integer, T> fn); static ID NONE; }### Answer:
@Test public void is_defined_null() { assertFalse(ID.isDefined(null)); }
@Test public void is_defined_none() { assertFalse(ID.isDefined(ID.NONE)); }