Java泛型、通配符、集合: compilation error

Question

给定以下类(class):

import java.util.ArrayList;
import java.util.Collection;

public class Main {

    private static class A {
    }

    private static class B<T> {
        private void thenReturn(T value) {
        }
    }

    private static <T> B<T> when(T methodCall) {
        return new B<T>();
    }

    private static Collection<? extends A> method() {
        return new ArrayList<>();
    }

    public static void main(String[] args) {
        Collection<? extends A> result = new ArrayList<>();
        // Does not compile.
        when(method()).thenReturn(result);
    }

}

我收到编译错误 The method thenReturn(Collection<capture#1-of ? extends Main.A>) in the type Main.B<Collection<capture#1-of ? extends Main.A>> is not applicable for the arguments (Collection<capture#3-of ? extends Main.A>)

我需要在 main 方法中更改哪些内容才能编译？有没有更好的解决方案

    public static void main(String[] args) {
        Collection result = new ArrayList<>();
        when(method()).thenReturn(result);
    }

Answer 1

这可以解决这个问题 - 看起来捕获规则对于长表达式来说有点紧张。

    Collection<? extends A> result = new ArrayList<>();
    B<Collection<? extends A>> when = when(method());
    when.thenReturn(result);