Getting null value after Mocking the function

Question

class Foo {
    ReturnParameters function1(int a, int b) {
        function2(a, b);
    }

    ReturnParameters function2(int a , int b) {
        // body of this function...
   }
}

I am creating a Junit test with Mockito and only want to test function1() and want to mock the returns from function2()

Below is my TestCase Class

class FooTest{
    Foo foo = mock(Foo.class);

    @Test
    public void function1test(){
        Mockito.when(foo.function2(1,2)).thenReturn(new ReturnParameters(100));
        ReturnParameters actualResult = foo.function1(1,2);
        int expectedResult = 100;
        AssertEquals(expectedResult , actualResult.getStatus());
    }
}

I am getting this error message that actualResult is a null value.

Can you please help?

Answer 1

You are attempting to partially stub the methods in a class. In order to do this, you must use Spy instead of Mock.

Here is some sample code (that assumes you are using Junit5 and uses doReturn.when instead of when.thenReturn):

@ExtendWith(MockitoExtension.class)
class TestFoo
{
    @Spy
    private Foo spyClassToTest;
    
    @BeforeEach
    public void before()
    {
        // stub the function 2 call.  The function 1 call is still "live"
        doReturn(new ReturnParameters(100)).when(spyClassToTest).function2(1, 2);
    }

    @Test
    public void function1_descriptionOfTheTestScenario_returns100()
    {
        ReturnParameters actualResult;
        int expectedResult = 100;

        // Perform the test
        actualResult = spyClassToTest.function1(1, 2);
        
        assertNotNull(actualResult);
        assertEquals(expectedResult, actualResult.getStatus());
    }
}

Answer 2

I don't know the domain but maybe its a good idea to split your functions into different classes. It should solve this issue in a clean way. Spy should also work though.

Answer 3

You can partially mock a class by using @Spy as mentioned in the other answer by DwB .

Or you can just provide a default answer when invoking mock() :

// import org.mockito.Answers;
Foo foo = mock(Foo.class, Answers.CALLS_REAL_METHODS);

This also works when using the annotations:

@Mock(answer = Answers.CALLS_REAL_METHODS)
Foo foo;