泛型和比较器

Question

I am working on generics and found that the following code is giving compile time error at comparing method .

Multiple markers at this line - Cannot infer type argument(s) for comparing(Function) - The type A does not define m1(Object) that is applicable here

 class A<T> {
    String m1() {
        return null;
    }
}

class B {
    void test() {
        Comparator<A<String>> target = Comparator.comparing(A::m1).thenComparing(A::m1);
    }
}

Can some one help me understand this behavior; and how can I fix the problem?

Answer 1

If you specify the exact generic types at the comparing method, the code compiles.

Comparator<A<String>> target =
    Comparator.<A<String>, String>comparing(A::m1).thenComparing(A::m1);

Answer 2

您应该为A类指定类型参数。

Comparator<A<String>> target = Comparator.comparing(A<String>::m1).thenComparing(A<String>::m1);

Answer 3

Interesting question. Haven't gone into JLS but I guess type inference does not work in case of chained method call. (You can see it works for a simple Comparator<A<String>> target = Comparator.comparing(A<String>::m1); )

One quick fix, which is similar to another answer, is help Java do the type inference:

Comparator<A<String>> target = Comparator.comparing(A<String>::m1)
                                         .thenComparing(A::m1);

Please note doing it at the first method already do the trick.

(Looking forward to see if someone can dig out JLS to see if such inference is supposed to be valid :P )

Answer 4

you can nested as

 Comparator<A<String>> target1 = Comparator.comparing(A::m1);
 Comparator<A<String>> target2 = target1.thenComparing(A::m1);


 myVarList.sort(target2);

Answer 5

 Comparator<A<String>> target = Comparator.comparing(A::m1).thenComparing(A::m1); 

thenComparing()期望Comparator对象作为参数...