泛型和通配符在Java

Question

集合在一個測試類使用AssertJ，我也有類似的代碼如下：

public void someTest() { 
    assertThat(getNames()).has(sameNamesAs(getExpectedNames())); 
    assertThat(getNames()).doesNotHave(sameNamesAs(getOtherNames())); 
} 

private List<String> getNames() { 
    return null; 
} 
private List<String> getExpectedNames() { 
    return null; 
} 
private List<String> getOtherNames() { 
    return null; 
} 

private Condition<List<String>> sameNamesAs(List<String> rhs) { 
    return new Condition<List<String>>("same names as " + rhs) { 
     @Override 
     public boolean matches(final List<String> lhs) { 
      return lhs.containsAll(rhs) && rhs.containsAll(lhs); 
     } 
    }; 
} 

我上調用編譯錯誤has和doesNotHave：

has/doesNotHave 
(org.assertj.core.api.Condition<? super java.util.List<? extends java.lang.String>>) 
in AbstractListAssert cannot be applied 
to 
(org.assertj.core.api.Condition<java.util.List<java.lang.String>>). 

我是新來的Java和我不明白的問題：java.util.List是超級類型的java.util.List和java.lang.String延伸java.lang.String，不是嗎？

Answer 1

在你的情況下，has和doesNotHave方法採取Condition<? super List<? extends T>條件，而不是作爲Condition<? super List<T>>你是從你的Condition<List<T>> sameNamesAs方法返回。

您需要Condition<List<? extends String>>類型的實例（它的原始類型Condition<? super List<? extends String>>的子類）：

private Condition<List<? extends String>> sameNamesAs(List<String> rhs) { 
    return new Condition<List<? extends String>>("same names as " + rhs) { ... }; 
} 

我試圖說明這與下面的代碼片段：

List<String> list = getNames(); 

// ELEMENT = String, ACTUAL = List<? extends ELEMENT> 
ListAssert<String> assertThat = assertThat(list); 

// by the signature, we have to pass Condition<? super ELEMENT> or Condition<? super ACTUAL> 
// Condition<? super ACTUAL> = Condition<? super List<? extends String>> 
Condition<List<? extends String>> condition = sameNamesAs(list); 

// Condition<List<? extends String>> extends Condition<? super List<? extends String>> 
assertThat.has(condition);