Getting started with mock objects

A mock object is a combination of a spy and a stub. It acts as an indirect output for a code under test, such as a spy, and can also stub methods to return values or throw exceptions, like a stub. A mock object fails a test if an expected method is not invoked or if the parameters of the method don't match.

The following steps demonstrate the test failure scenario:

1. Launch Eclipse, open <work_space>, and go to the 3605OS_TestDoubles project.
2. Create a com.packt.testdoubles.mock package and a StudentService class. This class will act as a course register service. The following is the code for the StudentService class:

   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);
     }
   }

3. Copy the StudentServiceSpy class and rename it as StudentServiceMockObject. Add a new method to verify the method invocations:

   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");
     }
   }

4. Modify the StudentService code to set the mock object, as we did in the spy example:

   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
   }

5. Create a test to verify the method invocation:

   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);
   
     }
   
   }

6. Run the test; it will fail, and you will get a verification error. The following screenshot shows the JUnit failure output:
   

The Mockito framework provides an API for mocking objects. It uses proxy objects to verify the invocation and stub calls.