Getting started with mock objects

public class StudentService {

  private Map<String, List<Student>> studentCouseMap = new HashMap<>();

  public void enrollToCourse(String courseName,Student student){
    List<Student> list = studentCouseMap.get(courseName);
    if (list == null) {
      list = new ArrayList<>();
    }

    if (!list.contains(student)) {
      list.add(student);
    }

    studentCouseMap.put(courseName, list);
  }
}

public void verify(String methodName, int numberOfInvocation){
  int actual = invocation(methodName);
  if(actual != numberOfInvocation){
    throw new IllegalStateException(methodName+" was expected ["+numberOfInvocation+"] times but actuallyactaully invoked["+actual+"] times");
  }
}

private StudentServiceMockObject mock;

public void setMock(StudentServiceMockObject mock) {
  this.mock = mock;
}
public void enrollToCourse(String courseName,Student student){
  MethodInvocation invocation = new MethodInvocation();

  invocation.addParam(courseName).addParam(student).setMethod("enrollToCourse");

  mock.registerCall(invocation);
  …//existing code
}

public class StudentServiceTest {
  StudentService service = new StudentService();
  StudentServiceMockObject mockObject = new StudentServiceMockObject();

  @Test
  public void enrolls_students() throws Exception {
    //create 2 students
    Student bob = new Student("001", "Robert Anthony");
    Student roy = new Student("002", "Roy Noon");

    //set mock/spy
    service.setMock(mockObject);

    //invoke method twice
    service.enrollToCourse("english", bob);
    service.enrollToCourse("history", roy);

    //assert that the method was invoked twice
    assertEquals(2, 
    mockObject.invocation("enrollToCourse"));

    //verify wrong information, that enrollToCourse was //invoked once, but actually it is invoked twice
    mockObject.verify("enrollToCourse", 1);

  }

}