Practice: Mockito

Question #1: Fill in the blanks.
A mocked method that is not stubbed will return ______ for objects, ______ for primitives, and an ______ collection for lists/maps.

Answer: A mocked method that is not stubbed will return null for objects, 0/false for primitives, and an empty collection for lists/maps.

Question #2: Explain the difference between @Mock and @Spy.
Answer:

Question #3: Fill in the missing line so this test passes by stubbing the mock:

@Test
void returnsHello() {
    MyRepo repo = mock(MyRepo.class);

    // TODO: stub repo.fetchMessage() to return "Hello"

    assertEquals("Hello", repo.fetchMessage());
}

Answer:

when(repo.fetchMessage()).thenReturn("Hello");

Question #4: Explain the difference between @Mock and @Spy.

Answer:
@Mock = everything is fake.
@Spy = real object + optional stubbing.

Question #5: When would a developer use @Mock instead of using the mock() API directly?

Answer:
Use @Mock when you want cleaner tests, automatic setup, and automatic injection. It keeps tests cleaner and easier to read, especially when multiple mocks are involved. @Mock requires @ExtendWith(MockitoExtension.class) and is more heavy-handed.

The mock(Logger.class) API is useful for creating mocks inside a method, not as fields. It can be clearer in small, simple test classes.