Is it possible to use method reference with static method of interface?

Adryr83 :

This is my first code:

public class MethodReference {
    public static void main (String []args) {
        Stream<String> s = Stream.of("brown bear", "grizzly");
        s.sorted(Comparator.reverseOrder()).forEach(System.out::print);
        //...
    }
}

Result: grizzlybrown bear

This is my second code:

public class MethodReference {
    public static void main (String []args) {
        Stream<String> s = Stream.of("brown bear", "grizzly");
        s.sorted(Comparator::reverseOrder()).forEach(System.out::print);
        //...
    }
}

Result: compiler error

My questions: Why is there a compiler error in the second code? Can't I use the method reference for static method of functional interface?

I know I can't use method reference with default method of functional interface. I know I can use method reference with a class in 5 cases:

Class

• Class::staticMethod

• Class::instanceMethod

• instance::instanceMethod

• Class::new

Functional Interface

• Interface::abstractMethod

Thanks a lot!

Michael :

Comparator.reverseOrder() is an expression which resolves to the Comparator type, because that's what it returns.

Comparator::reverseOrder is an expression which resolves to a method which takes no arguments and returns a Comparator e.g. a Supplier<Comparator<String>>, though it could be any matching functional interface.

In the second instance you are trying to pass a method (which provides a Comparator) as an argument. The method doesn't want that - it just wants the Comparator itself.

You could think of it like this (just pseudo-code to demonstrate the point):

s.sorted(new Comparator())

vs

s.sorted(new Supplier(new Comparator()))

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To answer your second question as to whether it's ever possible to use a method reference for a static method of an interface - yes, absolutely!

If we declare the following method:

<T> void giveMeAComparatorSupplier(Supplier<Comparator<T>> supplier) { }

then we can definitely call it with a method reference

giveMeAComparatorSupplier(Comparator::reverseOrder);

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(And FYI your method reference syntax is wrong - it never uses ())