Java泛型，通配符，集合：編譯錯誤

Question

考慮下面的類：

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); 
    } 

} 

我得到的編譯錯誤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，以便它會編譯改變？有沒有比

更好的解決方案

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

Answer 1

這有效解決它 - 它看起來像捕獲規則得到了長時間表達拉伸。

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