Java 泛型：<？> vs capture<？>

Question

我试图嘲笑ScheduledExecutorService，我在泛型方面遇到了问题。

下面是一个片段：

ScheduledFuture<?> future = mock(ScheduledFuture.class);
ScheduledExecutorService scheduledExecutorService = 
Mockito.mock(ScheduledExecutorService.class);
when(scheduledExecutorService.schedule(Mockito.any(Runnable.class), anyLong(), any(TimeUnit.class))).thenReturn(future);

这不会编译并显示以下错误：

Error:(20, 109) java: no suitable method found for 
  thenReturn(java.util.concurrent.ScheduledFuture<capture#1 of ?>)
method org.mockito.stubbing.OngoingStubbing.thenReturn(java.util.concurrent.ScheduledFuture<capture#2 of ?>) is not applicable
  (argument mismatch; java.util.concurrent.ScheduledFuture<capture#1 of ?> cannot be converted to java.util.concurrent.ScheduledFuture<capture#2 of ?>)
method org.mockito.stubbing.OngoingStubbing.thenReturn(java.util.concurrent.ScheduledFuture<capture#2 of ?>,java.util.concurrent.ScheduledFuture<capture#2 of ?>...) is not applicable
  (argument mismatch; java.util.concurrent.ScheduledFuture<capture#1 of ?> cannot be converted to java.util.concurrent.ScheduledFuture<capture#2 of ?>)

如果我声明ScheduledFuture没有泛型，它会编译并带有警告。

 ScheduledFuture future 

有什么办法可以正确吗？我的意思是，我可以使用任何通配符，以便在没有警告的情况下进行编译？

Answer 1

以下方法在类型上正确工作：

<T> ScheduledFuture<?> thenReturnMockFuture(
    OngoingStubbing<ScheduledFuture<T>> stubbing) {
  // Declare a local abstract class, so that future is type-correct.
  abstract class ScheduledFutureT implements ScheduledFuture<T> {}
  ScheduledFuture<T> future = mock(ScheduledFutureT.class);
  stubbing.thenReturn(future);
  return future;
}

然后像这样打电话：

ScheduledExecutorService scheduledExecutorService = 
    mock(ScheduledExecutorService.class);
ScheduledFuture<?> future = 
    thenReturnMockFuture(
        when(scheduledExecutorService.schedule(...)));

---

我认为你也可以这样做（没有方法，但使用抽象的本地类）：

ScheduledFuture<?> future = mock(ScheduledFutureT.class);
doReturn(future).when(scheduledExecutorService).schedule(...);

请注意doReturn不是类型安全的警告，因此您必须确保调用的方法是返回ScheduledFuture<?>。

Answer 2

ScheduledFuture中绑定的通配符会导致类型推断出现问题。您需要将future声明为 ScheduledFuture<?> 吗？

如果没有，则以下内容将起作用：

ScheduledFuture future = Mockito.mock(ScheduledFuture.class);
ScheduledExecutorService scheduledExecutorService =
        Mockito.mock(ScheduledExecutorService.class);
Mockito.when(scheduledExecutorService.schedule(Mockito.any(Runnable.class), Mockito.anyLong(), Mockito.any(TimeUnit.class))).thenReturn(future);

如果确实需要将future声明为 ScheduledFuture<?>则以下内容将起作用，因为doReturn()不是类型安全的：

ScheduledFuture<?> future = Mockito.mock(ScheduledFuture.class);
ScheduledExecutorService scheduledExecutorService =
        Mockito.mock(ScheduledExecutorService.class);
Mockito.doReturn(future).when(scheduledExecutorService).schedule(Mockito.any(Runnable.class), Mockito.anyLong(), Mockito.any(TimeUnit.class));