小编Jia*_* Li的帖子

对于Comparator类中的比较源代码

    public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
        Function<? super T, ? extends U> keyExtractor)
    {
      Objects.requireNonNull(keyExtractor);
      return (Comparator<T> & Serializable) (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
    }

我明白之间的差别super和extends.我不明白的是,为什么这种方法有它们.有人能举例说明参数看起来像什么时无法实现的Function<T, U> keyExtractor？

例如 :

Comparator<Employee> employeeNameComparator = Comparator.comparing(Employee::getName);

也可以使用以下函数定义进行编译

public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
    Function<T, U> keyExtractor)
{
  Objects.requireNonNull(keyExtractor);
  return (Comparator<T> & Serializable) (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
}