stubTest.java

public class GradesServiceTest {
    private Student student;
    private Gradebook gradebook;

    @Before
    public void setUp() throws Exception {
        gradebook = mock(Gradebook.class);
        student = new Student();
    }

    @Test
    public void calculates_grades_average_for_student() {
        when(gradebook.gradesFor(student)).thenReturn(grades(8, 6, 10)); //stubbing gradebook
        double averageGrades = new GradesService(gradebook).averageGrades(student);
        assertThat(averageGrades).isEqualTo(8.0);
    }
}