java-8 - 流 : "cannot convert Comparator to int" when using method reference

Question

假设我有一个字符串流:

Stream<String> stream = Stream.of("c","a","b");

这工作正常:

stream.sorted(Comparator.reverseOrder())

但使用替代语法不会:

stream.sorted(Comparator::reverseOrder)
                        ^
   Bad return type in method reference: cannot convert Comparator<T> to int

当我在 IDE 中按 Ctrl+左键单击时，两者都会将我带到相同的静态方法，返回 Comparator 中的比较器类(class):

  public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
        return Collections.reverseOrder();
    }

一本书解释说:

Comparator is a functional interface. This means that we can use method references or lambdas to implement it. The Comparator interface implements one method that takes two String parameters and returns an int . However, Comparator::reverseOrder doesn’t do that. It is a reference to a function that takes zero parameters and returns a Comparator . This is not compatible with the interface. This means that we have to use a method and not a method reference.

但我不明白。

Answer 1

方法引用等价于 Lambda 表达式，只能在 FunctionalInterface 处使用。是期待。 sorted不期望 FunctionalInterface ，它期望一个普通的，'ol Comparator .如果sorted的签名如下，方法引用会起作用:<T> sorted(Supplier<? extends Comparator<T>> s) .