Using Mockito to mock classes with generic parameters

Question

Is there a clean method of mocking a class with generic parameters? Say I have to mock a class Foo<T> which I need to pass into a method that expects a Foo<Bar>. I can do the following easily enough:

Foo mockFoo = mock(Foo.class);
when(mockFoo.getValue).thenReturn(new Bar());

Assuming getValue() returns the generic type T. But that's going to have kittens when I later pass it into a method expecting Foo<Bar>. Is casting the only means of doing this?

Answer 1

I think you do need to cast it, but it shouldn't be too bad:

Foo<Bar> mockFoo = (Foo<Bar>) mock(Foo.class);
when(mockFoo.getValue()).thenReturn(new Bar());

Answer 2

One other way around this is to use @Mock annotation instead. Doesn't work in all cases, but looks much sexier :)

Here's an example:

@RunWith(MockitoJUnitRunner.class)
public class FooTests {

    @Mock
    public Foo<Bar> fooMock;
    
    @Test
    public void testFoo() {
        when(fooMock.getValue()).thenReturn(new Bar());
    }
}

The MockitoJUnitRunner initializes the fields annotated with @Mock.

Answer 3

You could always create an intermediate class/interface that would satisfy the generic type that you are wanting to specify. For example, if Foo was an interface, you could create the following interface in your test class.

private interface FooBar extends Foo<Bar>
{
}

In situations where Foo is a non-final class, you could just extend the class with the following code and do the same thing:

public class FooBar extends Foo<Bar>
{
}

Then you could consume either of the above examples with the following code:

Foo<Bar> mockFoo = mock(FooBar.class);
when(mockFoo.getValue()).thenReturn(new Bar());

Answer 4

Create a test utility method. Specially useful if you need it for more than once.

@Test
public void testMyTest() {
    // ...
    Foo<Bar> mockFooBar = mockFoo();
    when(mockFooBar.getValue).thenReturn(new Bar());

    Foo<Baz> mockFooBaz = mockFoo();
    when(mockFooBaz.getValue).thenReturn(new Baz());

    Foo<Qux> mockFooQux = mockFoo();
    when(mockFooQux.getValue).thenReturn(new Qux());
    // ...
}

@SuppressWarnings("unchecked") // still needed :( but just once :)
private <T> Foo<T> mockFoo() {
    return mock(Foo.class);
}

Answer 5

I agree that one shouldn't suppress warnings in classes or methods as one could overlook other, accidentally suppressed warnings. But IMHO it's absolutely reasonable to suppress a warning that affects only a single line of code.

@SuppressWarnings("unchecked")
Foo<Bar> mockFoo = mock(Foo.class);

Answer 6

As the other answers mentioned, there's not a great way to use the mock() & spy() methods directly without unsafe generics access and/or suppressing generics warnings.

There is currently an open issue in the Mockito project (#1531) to add support for using the mock() & spy() methods without generics warnings. The issue was opened in November 2018, but there aren't any indications that it will be prioritized. Per one of the Mockito contributor's comments on the issue:

Given that .class does not play well with generics, I don't think there is any solution we can do in Mockito. You can already do @Mock (the JUnit5 extension also allows method parameter @Mocks) and that should be a suitable alternative. Therefore, we can keep this issue open, but it is unlikely ever to be fixed, given that @Mock is a better API.

Answer 7

Here is an interesting case: method receieves generic collection and returns generic collection of same base type. For example:

Collection<? extends Assertion> map(Collection<? extends Assertion> assertions);

This method can be mocked with combination of Mockito anyCollectionOf matcher and the Answer.

when(mockedObject.map(anyCollectionOf(Assertion.class))).thenAnswer(
     new Answer<Collection<Assertion>>() {
         @Override
         public Collection<Assertion> answer(InvocationOnMock invocation) throws Throwable {
             return new ArrayList<Assertion>();
         }
     });

Answer 8

With JUnit5 I think the best way is to @ExtendWith(MockitoExtension.class) with @Mock in the method parameter or the field.

The following example demonstrates that with Hamcrest matchers.

package com.vogella.junit5;                                                                    
                                                                                               
import static org.hamcrest.MatcherAssert.assertThat;                                           
import static org.hamcrest.Matchers.hasItem;                                                   
import static org.mockito.Mockito.verify;                                                      
                                                                                               
import java.util.Arrays;                                                                       
import java.util.List;                                                                         
                                                                                               
import org.junit.jupiter.api.Test;                                                             
import org.junit.jupiter.api.extension.ExtendWith;                                             
import org.mockito.ArgumentCaptor;                                                             
import org.mockito.Captor;                                                                     
import org.mockito.Mock;                                                                       
import org.mockito.junit.jupiter.MockitoExtension;                                             
                                                                                               
@ExtendWith(MockitoExtension.class)                                                            
public class MockitoArgumentCaptureTest {                                                      
                                                                                               
                                                                                               
    @Captor                                                                                    
    private ArgumentCaptor<List<String>> captor;                                               
                                                                                               
    @Test                                                                                      
    public final void shouldContainCertainListItem(@Mock List<String> mockedList) {            
        var asList = Arrays.asList("someElement_test", "someElement");                         
        mockedList.addAll(asList);                                                             
                                                                                               
        verify(mockedList).addAll(captor.capture());                                           
        List<String> capturedArgument = captor.getValue();                                     
        assertThat(capturedArgument, hasItem("someElement"));                                  
    }                                                                                          
}                                                                                              
                                                                                              

See https://www.vogella.com/tutorials/Mockito/article.html for the required Maven/Gradle dependencies.

Answer 9

JUnit5: use @ExtendWith(MockitoExtension.class) on the test class then add this field:

@Mock
Foo<Bar> barMock;

Answer 10

why not using spy

var mock = spy(new Foo<Bar>());
when(mockFoo.getValue()).thenReturn(new Bar());