Why different doubles serve different purposes in JUnit - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple stub for a dependency.

JUnit

MyService service = new MyService();
Dependency dep = new [1]();
service.setDependency(dep);

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

AMockDependency

BSpyDependency

CStubDependency

DFakeDependency

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing MockDependency which is used for interaction verification.

A stub provides fixed responses for testing without complex behavior.

2fill in blank

medium

Complete the code to verify that a method was called once using a mock.

JUnit

MockedList list = mock(MockedList.class);
list.add("item");
verify(list, [1]).add("item");

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) which expects two calls.

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

3fill in blank

hard

Fix the error in the spy creation code.

JUnit

List<String> list = new ArrayList<>();
List<String> spyList = [1](list);
spyList.add("test");

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

Amock

Bfake

Cstub

Dspy

Attempts:

3 left

💡 Hint

Common Mistakes

Using mock() which does not wrap the real object.

spy() wraps a real object to track interactions while keeping behavior.

4fill in blank

hard

Fill both blanks to create a fake that simulates a database connection and returns a fixed value.

JUnit

class FakeDatabase implements Database {
    public String query(String sql) {
        return [1];
    }
}

Database db = new FakeDatabase();
String result = db.query([2]);

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

A"fixed result"

B"SELECT * FROM users"

C"dynamic result"

D"DROP TABLE users"

Attempts:

3 left

💡 Hint

Common Mistakes

Returning dynamic result which is not fixed.

Using DROP TABLE which is destructive.

A fake provides a simple implementation returning fixed data. The query string is a typical SQL command.

5fill in blank

hard

Fill all three blanks to create a mock, set expectation, and verify interaction.

JUnit

List<String> mockList = [1](List.class);
when(mockList.get([2])).thenReturn("hello");
mockList.get(0);
verify(mockList).get([3]);

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

Amock

B0

Dspy

Attempts:

3 left

💡 Hint

Common Mistakes

Using spy() instead of mock().

Mismatching indexes in when() and verify().

mock() creates the mock object. The index 0 is used for both when() and verify() calls.

Practice

1. Which test double is primarily used to provide predefined data to the code under test without verifying interactions?

easy

A. Stub

B. Mock

C. Spy

D. Fake

Solution

1. Step 1: Understand the role of a stub

   A stub provides fixed data or responses to the code being tested but does not check if methods were called.

2. Step 2: Differentiate from mocks and spies

   Mocks verify interactions, spies do both spying and stubbing, and fakes are simplified implementations. Only stubs focus on providing data without verification.

3. Final Answer:

   Stub -> Option A

4. Quick Check:

   Stub = provides data only [OK]

Hint: Stubs give data, mocks check calls, spies do both [OK]

Common Mistakes:

• Confusing mocks with stubs
• Thinking spies only provide data
• Mixing fakes with stubs

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

easy

A. MyClass mock = spy(new MyClass());

B. MyClass mock = new MyClass();

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

D. MyClass mock = new FakeMyClass();

Solution

1. Step 1: Recall Mockito syntax for mocks

   Mockito.mock(Class.class) creates a mock object for the given class.

2. Step 2: Identify incorrect options

   MyClass mock = new MyClass(); creates a real object, not a mock. MyClass mock = spy(new MyClass()); creates a spy, not a mock. MyClass mock = new FakeMyClass(); uses a fake class, not a mock.

3. Final Answer:

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

4. Quick Check:

   Mockito.mock() creates mocks [OK]

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

Common Mistakes:

• Using new keyword instead of Mockito.mock
• Confusing spy() with mock()
• Assuming fake classes are mocks

3. Given the following code snippet using Mockito:

List<String> list = Mockito.spy(new ArrayList<>());
list.add("one");
Mockito.verify(list).add("one");
System.out.println(list.size());

What will be printed?

medium

A. 0

B. null

C. Throws an exception

D. 1

Solution

1. Step 1: Understand spy behavior

   A spy wraps a real object and records interactions but calls real methods unless stubbed.

2. Step 2: Analyze the code execution

   Adding "one" to the spy list calls the real add method, so the list size becomes 1. The verify confirms the add call happened.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   Spy calls real methods and records calls [OK]

Hint: Spy calls real methods and tracks calls [OK]

Common Mistakes:

• Thinking spy does not call real methods
• Assuming size remains 0 after add
• Expecting null or exception output

4. Identify the error in this Mockito test code:

MyService service = Mockito.mock(MyService.class);
service.process();
Mockito.verify(service.process());

medium

A. Missing MockitoAnnotations.initMocks(this) call

B. verify() should receive a method call without parentheses

C. verify() should be called before process()

D. process() method cannot be mocked

Solution

1. Step 1: Understand Mockito verify syntax

   Mockito.verify() expects the mock object as argument, then the method call without parentheses inside verify.

2. Step 2: Identify the mistake in code

   The code calls verify(service.process()), which calls process() immediately and passes its result. Correct is verify(service).process().

3. Final Answer:

   verify() should receive a method call without parentheses -> Option B

4. Quick Check:

   Use verify(mock).method() syntax [OK]

Hint: Use verify(mock).method() without parentheses inside verify [OK]

Common Mistakes:

• Calling method inside verify() parentheses
• Calling verify before method execution
• Confusing mock initialization with verification

5. You want to test a method that calls an external payment service. You need to check that the payment method was called with correct parameters, but also want to return a fixed success response. Which test double should you use?

hard

A. Mock

B. Spy

C. Stub

D. Fake

Solution

1. Step 1: Understand requirements for the test double

   You want to verify the method call (like a mock) and also provide a fixed response (like a stub).

2. Step 2: Identify the double that combines both features

   A mock verifies interactions and can return fixed responses when configured with when(mock.method(params)).thenReturn(success).

3. Final Answer:

   Mock -> Option A

4. Quick Check:

   Mock = verify calls + stub data [OK]

Hint: Mock verifies calls and stubs responses [OK]

Common Mistakes:

• Using stub which can't verify calls
• Using spy which executes real methods unless stubbed
• Confusing fake with mock