How to mock a generic parameter for a unit test in Java?

Question

I have a function signature I'd like to mock of an External Service.

public <T> void save(T item, AnotherClass anotherClassObject);

Given this function signature, and class name IGenericService how could one mock it with PowerMock? Or Mockito?

For this generic, I'm using: Class Theodore for the T in T item. For example, I tried using:

doNothing().when(iGenericServiceMock.save(any(Theodore.class),
                    any(AnotherClass.class));

IntelliJ cranks this:

save(T, AnotherClass) cannot be applied to 
(org.Hamcrest.Matcher<Theodore>, org.Hamcrest.Matcher<AnotherClass>)

And it cites the following reason:

reason: No instance(s) of type variable T exist 
so that Matcher<T> conforms to AnotherClass

First, the issue ought to solve if the generics argument is handled properly. What are some things one could do in such situations?

UPDATE: As ETO shared:

doNothing().when(mockedObject).methodToMock(argMatcher); 

shares the same fate.

Answer 1

You are passing wrong parameters to when. It may be a bit confusing, but there are two different usages of when method (actually those are two different methods):

1. when(mockedObject.methodYouWantToMock(expectedParameter, orYourMatcher)).thenReturn(objectToReturn);
   

2. doReturn(objectToReturn).when(mockedObject).methodYouWantToMock(expectedParameter, orYourMatcher);
   

Note: pay attention to input parameters of when method in both cases.

In your particular case you could do something like this:

doReturn(null).when(iGenericServiceMock).save(any(Theodore.class), any(AnotherClass.class));

This will fix your compilation issues. However the test will fail at runtime with org.mockito.exceptions.misusing.CannotStubVoidMethodWithReturnValue because you are trying to return something from a void method (null is not void). What you should do is :

doNothing().when(iGenericServiceMock).save(any(Theodore.class), any(AnotherClass.class));

Later you can check the interactions with your mock using verify method.

UPDATE:

Check your imports. You should use org.mockito.Matchers.any instead of org.hamcrest.Matchers.any.

Answer 2

Try to use the Mockito's ArgumentMatcher. Also in when put only the mock's reference:

doReturn(null).when(iGenericServiceMock).save(
    ArgumentMatchers.<Theodore>any(), ArgumentMatchers.any(AnotherClass.class));