Unit Testing Java Code - Mocking a non-static method of a different class

Question

public class First {

    public First(){
    }

    public String doSecond(){
        Second second = new Second();
        return second.doJob();
    }
}

class Second {

    public String doJob(){
        return "Do Something";
    }
}

Here I want to test the method "doSecond()" of class "First". For the same, I want to mock the method "doJob" of class "Second".

I know that I can create a mocked instance of class "Second" using the code below.

Second sec = mock(Second.class);
when(sec.doJob()).thenReturn("Stubbed Second");

But I cannot relate this mocked instance with class "First" as of the current code. Without refactoring the source code, is there any way by which i can achieve the requirement. Please help.

Answer 1

It's tricky to mock an instance that you create inside of a method, but it's possible.

Using PowerMock, you can accomplish this with the PowerMock.expectNew() method:

@RunWith(PowerMockRunner.class)
@PrepareForTest(First.class)
public class StackOverflowTest {

    @Test
    public void testFirst() throws Exception {
        Second secondMock = EasyMock.createMock(Second.class);
        PowerMock.expectNew(Second.class).andReturn(secondMock);
        expect(secondMock.doSecond()).andReturn("Mocked!!!");
        PowerMock.replay(secondMock, Second.class);
        String actual = new First().doSecond();
        PowerMock.verify(secondMock, Second.class);
        assertThat(actual, equalTo("Mocked!!!"));
    }

}

Effectively, PowerMock is proxying the creation of the new object and substituting whatever value we want when we invoke doSecond().

So, it's possible. However, this is a terrible practice to get into.

One typically wants to mock objects if they involve an outside concern, such as another layer (i.e. database, validation), or if the desired output is coming from other objects that are injected but are safe enough to consider tested.

If your method is capable of getting or retrieving data from a non-injectable source, you should not want to mock that out.

Considering that your method is simple and straightforward, you should really not need to do any mocks here at all. But if you felt that you were forced to, you could do one of a few things:

• Create a factory for the creation of Second, and mock the results of the returning factory object with Mockito.

• Pass in an instance of Second to that method, and use Mockito as the mock instance.

• Declare it as a field (i.e. injected dependency), and use Mockito.