Why mocking isolates units under test in JUnit - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a mock object using Mockito.

JUnit

UserService userService = Mockito.[1](UserService.class);

Drag options to blanks, or click blank then click option'

Averify

Bspy

Cmock

Dwhen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'spy' instead of 'mock' creates a partial mock, not a full mock.

Using 'verify' or 'when' are for other purposes, not creating mocks.

Mockito.mock() creates a mock object that simulates the behavior of the real class.

2fill in blank

medium

Complete the code to stub a method call on the mock to return a specific value.

JUnit

Mockito.[1](userService.getUserName()).thenReturn("Alice");

Drag options to blanks, or click blank then click option'

Averify

Bwhen

Cmock

Dspy

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'verify' instead of 'when' confuses verification with stubbing.

Using 'mock' or 'spy' here is incorrect as they create or wrap objects.

Mockito.when() is used to specify what a mock should return when a method is called.

3fill in blank

hard

Fix the error in the verification statement to check if getUserName() was called once.

JUnit

Mockito.verify(userService, Mockito.[1]()).getUserName();

Drag options to blanks, or click blank then click option'

Atimes(1)

BatLeastOnce()

Cnever()

Dtimes(2)

Attempts:

3 left

💡 Hint

Common Mistakes

Using times(2) checks for two calls, which is incorrect here.

Using never() checks that the method was not called, which is wrong.

Using atLeastOnce() allows multiple calls, not exactly one.

Mockito.times(1) verifies that the method was called exactly once.

4fill in blank

hard

Fill both blanks to stub a method and verify it was called with a specific argument.

JUnit

Mockito.[1](userService.getUserById([2])).thenReturn(new User("Bob"));
Mockito.verify(userService).getUserById(5);

Drag options to blanks, or click blank then click option'

Awhen

Bverify

C5

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using verify() in the first blank is wrong because it is for checking calls, not stubbing.

Using 10 instead of 5 causes verification to fail.

Use when() to stub the method call with argument 5, then verify it was called with 5.

5fill in blank

hard

Fill all three blanks to create a mock, stub a method, and verify the call.

JUnit

UserRepository repo = Mockito.[1](UserRepository.class);
Mockito.[2](repo.findUserByEmail([3])).thenReturn(new User("Eve"));
Mockito.verify(repo).findUserByEmail("[e-mail]");

Drag options to blanks, or click blank then click option'

Amock

Bwhen

C"[e-mail]"

Dspy

Attempts:

3 left

💡 Hint

Common Mistakes

Using spy() instead of mock() creates a partial mock, not a full mock.

Not passing the correct email string causes verification to fail.

First, create a mock with mock(), then stub the method with when(), passing the email string as argument.

Practice

1. Why do we use mocking in JUnit tests?
Mocking helps to:

easy

A. Replace the entire application with a mock

B. Isolate the unit under test by replacing its dependencies with fake objects

C. Automatically generate test data without writing code

D. Make the code run slower to catch timing issues

Solution

1. Step 1: Understand the purpose of mocking

   Mocking replaces real dependencies with fake ones to isolate the code being tested.

2. Step 2: Identify the effect on tests

   This isolation makes tests faster and more reliable by controlling external factors.

3. Final Answer:

   Isolate the unit under test by replacing its dependencies with fake objects -> Option B

4. Quick Check:

   Mocking isolates code = D [OK]

Hint: Mocking isolates by faking dependencies [OK]

Common Mistakes:

• Thinking mocking slows tests down
• Believing mocking generates test data automatically
• Confusing mocking with replacing the whole app

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

easy

A. MyClass mock = new Mock(MyClass);

B. MyClass mock = new MyClass();

C. MyClass mock = mockito.create(MyClass);

D. MyClass mock = Mockito.mock(MyClass.class);

Solution

1. Step 1: Recall Mockito syntax for mocking

   Mockito uses the static method mock() with the class type as argument.

2. Step 2: Check each option

   Only MyClass mock = Mockito.mock(MyClass.class); uses the correct syntax: Mockito.mock(MyClass.class);

3. Final Answer:

   MyClass mock = Mockito.mock(MyClass.class); -> Option D

4. Quick Check:

   Mockito.mock(Class) syntax = A [OK]

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

Common Mistakes:

• Using new keyword instead of Mockito.mock
• Wrong method names like create()
• Incorrect class references without .class

3. Given the following JUnit test code using Mockito:

class ServiceTest {
  @Mock
  Repository repo;

  @InjectMocks
  Service service;

  @Test
  void testGetData() {
    when(repo.getData()).thenReturn("mocked data");
    String result = service.getData();
    assertEquals("mocked data", result);
  }
}

What is the main reason this test passes even if the real Repository is not connected to a database?

medium

A. Because repo is a mock that returns controlled data

B. Because the real Repository is used and connected

C. Because service.getData() ignores the repository

D. Because the test does not check any output

Solution

1. Step 1: Understand the role of @Mock and when()

   The repo is mocked to return "mocked data" when getData() is called.

2. Step 2: Analyze test behavior

   The test passes because the mock returns the expected string, independent of any real database connection.

3. Final Answer:

   Because repo is a mock that returns controlled data -> Option A

4. Quick Check:

   Mock returns controlled data = C [OK]

Hint: Mocks return set values, ignoring real dependencies [OK]

Common Mistakes:

• Assuming real database is used
• Thinking service ignores repository
• Believing test lacks assertions

4. Identify the error in this JUnit test using Mockito:

@Test
void testCalculate() {
  Calculator calc = Mockito.mock(Calculator.class);
  when(calc.add(2, 3)).thenReturn(5);
  int result = calc.add(2, 3);
  assertEquals(6, result);
}

medium

A. The expected value in assertEquals is incorrect

B. The mock is not created properly

C. The when() method is used incorrectly

D. The test method lacks @Mock annotation

Solution

1. Step 1: Review mock setup and behavior

   The mock is set to return 5 when add(2, 3) is called.

2. Step 2: Check assertion correctness

   The assertion expects 6, but the mock returns 5, causing failure.

3. Final Answer:

   The expected value in assertEquals is incorrect -> Option A

4. Quick Check:

   Assert expected matches mock return = B [OK]

Hint: Assert expected value must match mock return [OK]

Common Mistakes:

• Expecting wrong values in assertions
• Confusing mock creation syntax
• Missing @Mock annotation on mock fields

5. In a complex system, why is mocking dependencies important to isolate the unit under test?
Consider a service that calls a database and a web API. Which benefit does mocking these dependencies provide?

hard

A. It automatically fixes bugs in the database and API

B. It makes the service code run slower to simulate real conditions

C. It allows testing the service logic without relying on database or API availability

D. It replaces the need for writing unit tests

Solution

1. Step 1: Understand dependency isolation

   Mocking replaces real dependencies like database and API with controlled fake versions.

2. Step 2: Identify benefits for testing

   This isolation allows testing service logic reliably without depending on external systems' availability or state.

3. Final Answer:

   It allows testing the service logic without relying on database or API availability -> Option C

4. Quick Check:

   Mocking isolates external dependencies = A [OK]

Hint: Mock dependencies to test logic independently [OK]

Common Mistakes:

• Thinking mocking slows code
• Believing mocking fixes bugs automatically
• Confusing mocking with skipping tests