Mock objects in JUnit - Test Execution Trace

Test Overview

This test uses a mock object to simulate a dependency and verifies that the service method calls the dependency correctly and returns the expected result.

Test Code - JUnit with Mockito

JUnit

import static org.mockito.Mockito.*;
import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;

class UserServiceTest {

    interface UserRepository {
        String getUserEmail(int userId);
    }

    static class UserService {
        private final UserRepository userRepository;

        UserService(UserRepository userRepository) {
            this.userRepository = userRepository;
        }

        String getUserEmail(int userId) {
            return userRepository.getUserEmail(userId);
        }
    }

    @Test
    void testGetUserEmail() {
        // Create mock
        UserRepository mockRepo = mock(UserRepository.class);

        // Define behavior
        when(mockRepo.getUserEmail(1)).thenReturn("[e-mail]");

        // Inject mock into service
        UserService userService = new UserService(mockRepo);

        // Call method
        String email = userService.getUserEmail(1);

        // Verify result
        assertEquals("[e-mail]", email);

        // Verify interaction
        verify(mockRepo).getUserEmail(1);
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized	—	PASS
2	Creates mock object for UserRepository using Mockito.mock()	Mock UserRepository instance created	—	PASS
3	Defines mock behavior: when getUserEmail(1) is called, return '[e-mail]'	Mock configured to return fixed email for userId 1	—	PASS
4	Creates UserService instance with mock UserRepository injected	UserService ready with mock dependency	—	PASS
5	Calls userService.getUserEmail(1)	Method call routed to mock, returns '[e-mail]'	Returned email equals '[e-mail]'	PASS
6	Verifies that mockRepo.getUserEmail(1) was called exactly once	Mockito verifies interaction	Mock method getUserEmail(1) was called once	PASS
7	Test ends successfully	All assertions passed	—	PASS

Failure Scenario

Failing Condition: Mock method getUserEmail(1) not called or returns unexpected value

Execution Trace Quiz - 3 Questions

What does the mock object simulate in this test?

A The UserService class

B The JUnit test runner

C The UserRepository dependency

D The assertEquals method

Key Result

Using mock objects helps isolate the unit under test by simulating dependencies, allowing focused and reliable testing of behavior without relying on real implementations.

Practice

1. What is the main purpose of using mock objects in JUnit testing?

easy

A. To speed up the compilation of test classes

B. To simulate real objects and control their behavior during tests

C. To replace the JUnit framework with a simpler tool

D. To automatically generate test data without coding

Solution

1. Step 1: Understand the role of mock objects

   Mock objects imitate real objects to isolate the code being tested and control responses.

2. Step 2: Identify the main benefit in testing

   This helps test code without relying on actual implementations or external systems.

3. Final Answer:

   To simulate real objects and control their behavior during tests -> Option B

4. Quick Check:

   Mock objects = simulate and control behavior [OK]

Hint: Mocks simulate objects to isolate tests quickly [OK]

Common Mistakes:

• Thinking mocks speed up compilation
• Confusing mocks with test data generators
• Believing mocks replace the JUnit framework

2. Which of the following is the correct way to create a mock object using Mockito in JUnit?

easy

A. MyClass mock = mock(MyClass.class);

B. MyClass mock = new MyClass().mock();

C. Mock<MyClass> mock = new Mock<>();

D. MyClass mock = createMock(MyClass);

Solution

1. Step 1: Recall Mockito syntax for creating mocks

   Mockito uses mock(Class.class) to create mock objects.

2. Step 2: Match the syntax to options

   MyClass mock = mock(MyClass.class); matches the correct syntax: MyClass mock = mock(MyClass.class);

3. Final Answer:

   MyClass mock = mock(MyClass.class); -> Option A

4. Quick Check:

   Mockito mock creation = mock(Class.class) [OK]

Hint: Use mock(Class.class) to create mocks in Mockito [OK]

Common Mistakes:

• Using new Mock<>() which is not valid
• Calling mock() on an instance
• Using createMock which is not Mockito syntax

3. Given the following code snippet, what will result be after execution?

List<String> mockList = mock(List.class);
when(mockList.size()).thenReturn(5);
int result = mockList.size();

medium

A. 5

B. 0

C. Throws NullPointerException

D. Returns the actual size of a real list

Solution

1. Step 1: Understand the mock setup

   The mock list's size() method is stubbed to return 5 when called.

2. Step 2: Evaluate the method call

   Calling mockList.size() returns 5 as per the stub, not the real list size.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Stubbed method returns set value 5 [OK]

Hint: Stubbed methods return specified values, not defaults [OK]

Common Mistakes:

• Expecting default 0 for int return
• Thinking mock calls throw exceptions by default
• Confusing mock with real object behavior

4. Identify the error in the following JUnit test code using Mockito:

MyService service = mock(MyService.class);
when(service.getData()).thenReturn("data");
verify(service).getData();

medium

A. No error, code is correct

B. Incorrect use of mock() method

C. Cannot verify void methods

D. Missing call to service.getData() before verify

Solution

1. Step 1: Understand verify usage in Mockito

   Verification checks if a method was called on the mock during the test.

2. Step 2: Check if method was called before verify

   The code never calls service.getData() before verify, so verification fails.

3. Final Answer:

   Missing call to service.getData() before verify -> Option D

4. Quick Check:

   Verify requires method call before checking [OK]

Hint: Always call method before verify to avoid errors [OK]

Common Mistakes:

• Verifying without calling the method
• Confusing mock creation syntax
• Thinking verify only works on void methods

5. You want to test a method that calls a dependency twice with different inputs and expects different outputs. How do you configure a mock to return "first" on the first call and "second" on the second call to getValue()?

hard

A. when(mock.getValue()).thenReturn("first"); when(mock.getValue()).thenReturn("second");

B. when(mock.getValue()).thenReturn(new String[]{"first", "second"});

C. when(mock.getValue()).thenReturn("first").thenReturn("second");

D. when(mock.getValue()).thenReturn("second").thenReturn("first");

Solution

1. Step 1: Recall Mockito chaining for multiple returns

   Mockito allows chaining thenReturn() calls to specify sequential return values.

2. Step 2: Match chaining syntax to options

   when(mock.getValue()).thenReturn("first").thenReturn("second"); matches the correct chaining syntax for sequential calls.

3. Final Answer:

   when(mock.getValue()).thenReturn("first").thenReturn("second"); -> Option C

4. Quick Check:

   Chain thenReturn() for multiple call returns [OK]

Hint: Chain thenReturn() calls for sequential mock returns [OK]

Common Mistakes:

• Passing multiple values in one thenReturn() call
• Overwriting stubs instead of chaining
• Reversing the order of return values