[FIXED] Is it required to define doNothing()/when() on Objects mocked using Mockito

Issue

I am writing unit tests using JUnit5 and Mockito for a class Main.java.

The dependency class ExternalApi.java is injected in Main.java.

Here is my MainTest.java:

@ExtendWith(MockitoExtension.class)
public class MainTest {
    @InjectMocks
    Main main;

    @Mock
    ExternalApi externalApi;

    @Test
    void testAbcMethod() {

    }
}

The abc() method in Main.java calls externalApi.doSomething() which does some stuff.

I don't want this original externalApi method to be called in my test. Do I need to define

doNothing().when(externalApi).when(doSomething());

or is it just enough to mock ExternalApi?

What happens when we don't define what happens when we call a mocked object's methods?

Solution

What happens when we don't define what happens when we call a mocked object's methods?

Nothing happens. Mocking is enough since it's not the real class/method that is called but the mocked one.

If you had spied on the instance, that would have triggered the real method call. Only then, you'd have thought of using doNothing to silence the method behavior.

@Test
public void whenNotDefineBehaviorOnMock_thenCorrect() {
    List mockList = Mockito.mock(ArrayList.class);
    
   // No behavior defined,
    mockList.add("one");
    Mockito.verify(mockList).add("one");
   // Size is still empty
    assertEquals(0, mockList.size());

   // Behavior defined
    Mockito.when(mockList.size()).thenReturn(100);
    assertEquals(100, mockList.size());
}

Answered By - exaucae
Answer Checked By - Katrina (JavaFixing Volunteer)