java - 如何使 JUnit assertThat() 与下界通配符一起使用？

Question

最初，我有这段生产代码：

interface ActionSequence {
   public List<Actions> getActions();

我测试了实现该接口的类，如下所示：

assertThat(sequenceUnderTest.getActions(), is(Arrays.asList(action1, action2));

然后我认为将生产界面更改为：

public List<? extends Action> getActions() {

（允许我返回 Action 的子类列表）。

但现在日食告诉我：

The method assertThat(T, Matcher<? super T>) in the type Assert is not applicable for the arguments (List<capture#1-of ? extends Action>, Matcher<List<Action>>)

我想：当我更改实现 ActionInterface的类时

@Override
public List<SomeSubClassOfAction> getActions()

（而不是保留通配符）......然后一切正常。但为什么？

Answer 1

Arrays.asList(action1, action2)将返回一个List<Action>. is(Arrays.asList(action1, action2))因此将返回一个Matcher<List<Action>>.

assertThat 具有以下签名：

assertThat(T actual, Matcher<T> matcher)

所以 assertThat 在您的情况下需要以下参数：

assertThat(List<Action>, Matcher<List<Action>>)

但是您的第一个参数是 aList<? extends Action> 并且 a 与 aList<Action>完全不同List<? extends Action>。例如，您不能将 Action 元素放入List<SomeSubClassOfAction>. 这就是为什么这行不通。

详情请看Angelika Langer的优秀网站：http ://www.angelikalanger.com/GenericsFAQ/FAQSections/Index.html

Answer 2

Your question was, why

@Override
public List<SomeSubClassOfAction> getActions()

is a legal Implementation of

public List<? extends Action> getActions()

The Answer is covariant return. Since Java1.5 subclasses are allowed to specialize the return of inherited methods.

But I wouldn't recommend having wildcard types in return parameters because it isn't client friendly. See Generic wildcard types should not be used in return parameters and its quote from Effective Java