Mockito mock NullPointerException，不知道还有什么可以尝试的

Question

Java thinks my mocked Course class object is null at the line when().thenReturn().

class StudentTest {

    @Mock
    Course courseMock1;      

    @Test
    void student_getTeacherNames_should_return_list_of_full_names() {    
        when(courseMock1.getEAP()).thenReturn(1);
    }


public class Course {
    public Course(String courseName, String name, LocalDate startDate, LocalDate endDate, Integer EAP, Teacher teacher) {
        this.courseName = courseName;
        this.name = name;
        this.startDate = startDate;
        this.endDate = endDate;
        this.EAP = EAP;
        this.teacher = teacher;
    }
    public Integer getEAP() {
        return EAP;
    }
}

I have tried:

@RunWith(MockitoJUnitRunner.class)
class StudentTest{...

--

@Before
public void setup(){
    MockitoAnnotations.initMocks(this);
}

--

@Rule public Mocks mocks = new Mocks(this);

none of which solve NPE.

Also tried (using mocked Teacher object as one of the parameters)

@Mock
Course courseMock1 = new Course(params..);

which yielded: MissingMethodInvocationException: when() requires an argument which has to be 'a method call on a mock'

Answer 1

Annotate StudentTest class with @RunWith(MockitoJUnitRunner.class) .

Declare StudentTest class as public .

Declare student_getTeacherNames_should_return_list_of_full_names method as public .

Run the test class.

@RunWith(MockitoJUnitRunner.class)
public class StudentTest {

    @Mock
    Course courseMock1;      

    @Test
    public void student_getTeacherNames_should_return_list_of_full_names() {    
        Mockito.when(courseMock1.getEAP()).thenReturn(1);
        assertThat(courseMock1.getEAP()).isEqualTo(1);
    }
}

Answer 2

What test framework are you using? junit 4 or junit 5?

If junit5, then the test class should be annotated with @ExtendWith(MockitoExtension.class)

An alternative could be to rely purely on code (not using annotation). The code would become

class StudentTest {
    Course courseMock1 = Mockito.mock(Course.class);
    ...
}

Answer 3

To use @Mock annotations you need to import org.mockito.MockitoAnnotations; and call MockitoAnnotations.initMocks. Note that the method has to be called before each test.

@BeforeEach
void setup() {
    MockitoAnnotations.initMocks(this);
}

I used mistakenly @Before instead of @Before Each