Java泛型，通配符，集合：编译错误

Question

考虑下面的类：

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); 
    } 

} 

我得到的编译错误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>)

究竟我在main方法，以便它会编译改变？有没有比

更好的解决方案

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

Answer 1

这有效解决它 - 它看起来像捕获规则得到了长时间表达拉伸。

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