In Java 8, it is possible to refer to unary methods which return a boolean/Boolean (e.g. Collection.contains(Object)) using method reference syntax, returning an equivalent Predicate object for the referent method:
final Set<Integer> acceptedValues = new HashSet<>();
final Predicate<Integer> isValueAccepted = acceptedValues::contains;
Moreover, the negation of this object can be retrieved by using Predicate.negate():
final Predicate<Integer> isValueNotAccepted = isValueAccepted.negate();
However, is there no shorthand for directly negating a method reference itself? — for example, the following expressions are not compilable:
final Predicate<Integer> isValueNotAccepted = acceptedValues::contains::negate;final Predicate<Integer> isValueNotAccepted = acceptedValues::contains.negate();final Predicate<Integer> isValueNotAccepted = !acceptedValues::contains;
