Java：<?>与capture<?>

Question

我试图嘲弄ScheduledExecutorService，我在仿制药方面遇到了麻烦。

这里有一个片段：

ScheduledFuture<?> future = mock(ScheduledFuture.class);

ScheduledExecutorService scheduledExecutorService = 
Mockito.mock(ScheduledExecutorService.class);

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

这不会使用以下错误进行编译：

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时没有泛型，那么它编译时会发出警告。

 ScheduledFuture future 

有什么办法可以正确地解决这个问题吗？我的意思是，我是否可以使用通配符来编译而不需要警告呢？

Answer 1

下列工作类型正确：

<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;
}

然后打电话给：

ScheduledExecutorService scheduledExecutorService = 
    mock(ScheduledExecutorService.class);

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

我认为您也可以这样做(不使用方法，而是使用抽象的本地类)：

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

注意到警告，doReturn并不是类型安全的，所以您只需要确保调用一个确实返回ScheduledFuture<?>的方法。

Answer 2

ScheduledFuture中的通配符绑定会导致类型推断问题。需要将future声明为ScheduledFuture<?>吗？

如果不这样做，以下几点将有效：

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);

如果您确实需要将future声明为ScheduledFuture<?>，那么由于doReturn()不是类型安全的，所以下面的操作将有效：

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));