Java Comparator 类型推断混淆

Question

我对以下代码示例几乎没有疑问。

class HypotheticComparators {
    class Pair<U,T> {
        private U left;
        private T right;
        public U getLeft() {
            return left
        }
        public T getRight() {
            return right;
        }
    }

    class User {
        private String name;
        private Integer age;
        public String getName(){
            return name;
        }
        public Integer getAge() {
            return age;
        }
    }

    public static void main(String[] args){
        List<Pair<LocalDate,LocalDate>> dataIntevals = new ArrayList<>();

        //Doesn't Compile
        Comparator<Pair<LocalDate,LocalDate>> pairComparator1  = Comparator.comparing(Pair::getLeft).thenComparing(Pair::getRight);

        //This Compile
        Comparator<Pair<LocalDate,LocalDate>> leftComparator = Comparator.comparing(Pair::getLeft);
        Comparator<Pair<LocalDate,LocalDate>> pairComparator2 = leftComparator.thenComparing(Pair::getRight));

        //This compile
        Comparator<User> userComparator1 = Comparator.comparing(User::getName).thenComparing(User::getAge);

        //This also compile
        Comparator<User> userNameComparator = Comparator.comparing(User::getName);
        Comparator<User> userComparator2 = userNameComparator.thenComparing(User::getAge);

    }
}

对于这个声明，Comparator<Pair<LocalDate,LocalDate>> pairComparator1 = Comparator.comparing(Pair::getLeft).thenComparing(Pair::getRight); , 我对它为什么不编译的最初想法是 themComparaing无法从 Comparator.comparing(Pair::getLeft) 推断类型.但我不相信这个想法，因为查看 Comparator.comparing 的源代码和 thenComparing

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));
}

default <U extends Comparable<? super U>> Comparator<T> thenComparing(
        Function<? super T, ? extends U> keyExtractor)
{
    return thenComparing(comparing(keyExtractor));
}

Comparator.comparing(Pair::getLeft)可以使用 T 作为 Pair<LocalDate, LocalDate> 进行类型推断, themComparing也可以使用 T 作为 Pair<LocalDate, LocalDate> 进行类型推断.此外，如果我的想法是正确的，为什么Comparator<User> userNameComparator = Comparator.comparing(User::getName);可以毫无问题地编译。我在这里遗漏了什么吗？

Answer 1

Java 类型推断器无法确定第一个 .comparing将接受与第二个方法相同的类型，因此它猜测第一个方法的类型是 Object, Pair<LocalDate, LocalDate> ，并且 Object 没有实现 Comparable。您可以通过为方法指定泛型类型来解决此问题:

Comparator<Pair<LocalDate,LocalDate>> pairComparator1  = Comparator
    .<Pair<LocalDate, LocalDate>, LocalDate>comparing(Pair::getLeft)
    .thenComparing(Pair::getRight);