Mockito 2 Mocks Returning Stream - No suitable method found

I have a function in the implementation with the signature

Class Foo {
    Stream<? extends Bar> doSomething() { ... }
}

However, in my tests when I did something like

when(foo.doSomething()).thenReturn(Stream.of(bar1, bar2));

I got the following compilation error:

no suitable method found for thenReturn(java.util.stream.Stream<Bar>)
method org.mockito.stubbing.OngoingStubbing.thenReturn(java.util.stream.Stream<capture#1 of ? extends Bar>,java.util.stream.Stream<capture#1 of ? extends Bar>...) is not applicable
      (argument mismatch; inference variable T has incompatible bounds
        equality constraints: capture#1 of ? extends Bar
          lower bounds: Bar)

Unfortunately, using Stream.<? extends Foo>(bar1, bar2) didn't work either. Looking for a fix.

Thanks!

EDIT:
FYI this worked when I was using Mockito 1.10.x, it stopped working once i updated Mockito to 2.28.x

Solution

The only possible workaround is using OngoingStubbing<T> thenAnswer(Answer<?> answer) which expects any parametrized type (Answer<?>):

• expanded:

  Stream<? extends Bar> stream = Stream.of(new Buzz(), new Buzz(), new Buzz());
  

  Mockito.when(foo.doSomething()).thenAnswer(new Answer<Stream<? extends Bar>>() {
      @Override
      public Stream<? extends Bar> answer(InvocationOnMock invocation) {
          return stream;
      }
  });
  

• short version:

  Mockito.when(foo.doSomething()).thenAnswer(invocation -> stream);
  

Explanation

The method OngoingStubbing<T> thenReturn(T value) expects in case of mocking Stream<? extends Bar> doSomething() method the Stream<? extends Bar> type at the input. Passing Stream.of(bar1, bar2)) doesn't guarantee the parameters fit the same parametrized type.

Let Buzz extend Bar:

Stream<? extends Bar> stream = Stream.of(new Buzz(), new Buzz(), new Buzz());
Mockito.when(foo.doSomething()).thenReturn(stream);

The very same issue is reproducible with the following code:

static class FluentFactory {
    static <T> Fluent<T> fluent(T methodCall) { return new Fluent<>(); }
}
    
static class Fluent<T>  {
    Fluent<T> first(T t) { return new Fluent<>(); }
}

Stream<? extends Bar> stream = Stream.of(new Buzz(), new Buzz(), new Buzz());
FluentFactory.fluent(foo.doSomething()).first(stream);

> Error:(xx, yy) java: incompatible types: java.util.stream.Stream<capture#1 of ? extends com.mycompany.Foo> cannot be converted to java.util.stream.Stream<capture#2 of ? extends com.mycompany.Foo>

A workaround to this, use:

when(fooMock.doSomething())
        .thenAnswer((Answer<Stream<? extends Bar>>) invocation -> Stream.of(bar1, bar2));

This causes no compilation or runtime error.