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 lt T gt  which I need to pass into a method that expects a Foo lt Bar gt    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 lt Bar gt    Is casting the only means of doing this

User · Answer

Create a test utility method  Specially useful if you need it for more than once    Test public void testMyTest                    Foo lt Bar gt  mockFooBar   mockFoo        when mockFooBar getValue  thenReturn new Bar          Foo lt Baz gt  mockFooBaz   mockFoo        when mockFooBaz getValue  thenReturn new Baz          Foo lt Qux gt  mockFooQux   mockFoo        when mockFooQux getValue  thenReturn new Qux                    SuppressWarnings  unchecked      still needed    but just once    private  lt T gt  Foo lt T gt  mockFoo         return mock Foo class

User · Answer

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 lt Bar gt  fooMock        Test     public void testFoo             when fooMock getValue    thenReturn new Bar               The MockitoJUnitRunner initializes the fields annotated with  Mock

User · Answer

Here is an interesting case  method receieves generic collection and returns generic collection of same base type  For example   Collection lt   extends Assertion gt  map Collection lt   extends Assertion gt  assertions     This method can be mocked with combination of Mockito anyCollectionOf matcher and the Answer   when mockedObject map anyCollectionOf Assertion class    thenAnswer       new Answer lt Collection lt Assertion gt  gt                Override          public Collection lt Assertion gt  answer InvocationOnMock invocation  throws Throwable                return new ArrayList lt Assertion gt

User · Answer

I think you do need to cast it  but it shouldn t be too bad   Foo lt Bar gt  mockFoo    Foo lt Bar gt   mock Foo class   when mockFoo getValue    thenReturn new Bar

User · Answer

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 lt List lt String gt  gt  captor                                                                                                                                                      Test                                                                                           public final void shouldContainCertainListItem  Mock List lt String gt  mockedList                        var asList   Arrays asList  quot someElement test quot    quot someElement quot                                     mockedList addAll asList                                                                                                                                                                        verify mockedList  addAll captor capture                                                        List lt String gt  capturedArgument   captor getValue                                                 assertThat capturedArgument  hasItem  quot someElement quot                                                                                                                                                                                                                                                                                                                                       See https   www vogella com tutorials Mockito article html for the required Maven Gradle dependencies

User · Answer

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 lt Bar gt  mockFoo   mock Foo class

User · Answer

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 lt Bar gt        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 lt Bar gt        Then you could consume either of the above examples with the following code   Foo lt Bar gt  mockFoo   mock FooBar class   when mockFoo getValue    thenReturn new Bar

User · Answer

As the other answers mentioned  there s not a great way to use the mock    amp  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    amp  spy   methods without generics warnings   The issue was opened in November 2018  but there aren t any indications whether it will be prioritized

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