What is the difference between Comparator::reverseOrder and Comparator.reverseOrder() when used in sorted method of stream

Question

What is the difference between Comparator::reverseOrder and Comparator.reverseOrder() when used in sorted method of stream.

    Stream<String> streamText = Stream.of("over the river", "through the woods", "to grandmother's house we go");

This works:

    streamText.filter(n -> n.startsWith("t"))
         .sorted(Comparator.reverseOrder())
         .findFirst().ifPresent(System.out::println);

But this does not compile:

    streamText.filter(n -> n.startsWith("t"))
         .sorted(Comparator::reverseOrder)
         .findFirst().ifPresent(System.out::println);

Answer 1

Good question!

sorted need a Comparator<T>, right? Comparator<T> is a functional interface. It represents a function that takes 2 arguments and returns an int indicating which argument is greater or whether they are equal.

In the case of Comparator.reverseOrder(), reverseOrder is a method that returns a Comparator<T>. In this case you call the method and it returns a Comparator that can be used as the parameter for sorted. Everything is good.

In the case of Comparator::reverseOrder, you are not calling reverseOrder. Instead, you are passing reverseOrder as a function into sorted. As mentioned above, sorted will accept a function that takes 2 parameters and returns an int, but you are giving it reverseOrder, which takes no arguments and returns a Comparator<T>. See the mismatch here?

Expected: a function that takes 2 parameters and returns an int OR a Comparator<T> object

What you gave it: a function that takes no parameters and returns a Comparator<T>

This results in a compiler error.