收集器.maxBy(比较器自然领主())无法编译,尽管推断了Long

我不能编译下面的代码(尝试 onlinegdb

List<Container<Dto>> list = Arrays.asList(
                new Container<>(new Dto("A"), 10L),
                new Container<>(new Dto("A"), 30L),
                new Container<>(new Dto("B"), 30L));

Map<String, Optional<Long>> mapWrong = list.stream()
    .collect(Collectors.groupingBy(
        c -> c.getOutput().getType(),
        Collectors.mapping(
            Container::getDifference, 
            Collectors.maxBy(Comparator.naturalOrder()))));     // DOESN'T WORK WITH THIS 

不兼容类型:无法推断类型变量T,U,A,R,capture#2 of?,T,T 找到:java.lang.Object

Collectors.mapping 将对象映射到新值 Container::getDifference 退货 long 同样的类型应该被推断出来 Collectors.maxBy

令人惊讶的是,当我替换 Comparator.naturalOrder() 具有 Comparator.comparingLong(l -> l) ,那么它就起作用了。

Map<String, Optional<Long>> mapCorrect = list.stream()
    .collect(Collectors.groupingBy(
        c -> c.getOutput().getType(),
        Collectors.mapping(
            Container::getDifference, 
            Collectors.maxBy(Comparator.comparingLong(l -> l)))));   // A CHANGE TO FIX IT

public class Container<T> {
    T output;
    long difference;
}

public class Dto {
    String type;
}

注意这是可重复使用OpenJDK和 jdk-11.0.5 和 jdk1.8.0_212