Stream sorted method with comparator

Question

I don't understand why code1 works but code2 doesn't compile. Please explain.

//Code1:
    Stream<String> s = Stream.of("AA", "BB");
    s.sorted(Comparator.reverseOrder())
            .forEach(System.out::print);

//Code2:
    Stream<String> s = Stream.of("AA", "BB");
    s.sorted(Comparator::reverseOrder)
            .forEach(System.out::print);

The difference between the two is code1 uses Comparator.reverseOrder() while code2 uses Comparator::reverseOrder

Answer 1

Because the first example is a factory-method so when you inspect it, you see that you get a comparator back.

But the second one is a method-reference which you could write like this:

Stream<String> s = Stream.of("AA", "BB");
s.sorted(() -> Comparator.reverseOrder()) // no semantic difference!
    .forEach(System.out::print);

But it has a whole different meaning because this time you are given Stream#sorted() a Supplier<Comparator<?>> but it just needs a Comparator<?>

Small Sidenote: Don't store streams in variables, use them directly. So i would suggest you just write:

Stream.of("AA", "BB")
    .sorted(Comparator.reverseOrder())
    .forEach(System.out::print);