Spy objects in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a spy of the List class using Mockito.

JUnit

List<String> spyList = Mockito.[1](new ArrayList<>());

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

Aspy

Bmock

Cverify

Dwhen

Attempts:

3 left

💡 Hint

Common Mistakes

Using mock() instead of spy() will create a mock without real method calls.

Using verify() or when() here is incorrect because they are for verification and stubbing.

Mockito.spy() creates a spy object that wraps a real instance, allowing you to track interactions while keeping original behavior.

2fill in blank

medium

Complete the code to verify that the add method was called once on the spyList.

JUnit

Mockito.[1](spyList).add("item");

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

Awhen

Bspy

Cverify

Dmock

Attempts:

3 left

💡 Hint

Common Mistakes

Using when() instead of verify() confuses stubbing with verification.

Using spy() or mock() here is incorrect because they create objects, not verify calls.

Mockito.verify() is used to check if a method was called on a mock or spy object.

3fill in blank

hard

Fix the error in stubbing the size method of spyList to return 5.

JUnit

Mockito.[1](5).when(spyList).size();

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

Awhen

Bverify

Cspy

DdoReturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using when(spyList.size()) calls the real method immediately, which can cause errors.

Using verify() or spy() here is incorrect for stubbing.

When stubbing spy methods, doReturn().when() is preferred to avoid calling the real method during stubbing.

4fill in blank

hard

Fill both blanks to stub the get method of spyList to return "mocked" for index 0.

JUnit

Mockito.[1]("mocked").when(spyList).[2](0);

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

AdoReturn

Bwhen

Cget

Dspy

Attempts:

3 left

💡 Hint

Common Mistakes

Using when() in the first blank causes the real method to be called during stubbing.

Using spy() or verify() in blanks is incorrect here.

doReturn(value).when(spy).method(args) is the correct pattern to stub spy methods. Here, get(0) is stubbed to return "mocked".

5fill in blank

hard

Fill all three blanks to verify that the clear method was called exactly twice on spyList.

JUnit

Mockito.[1](spyList, Mockito.[2](2)).[3]();

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

Averify

Btimes

Cclear

Dspy

Attempts:

3 left

💡 Hint

Common Mistakes

Using spy() in the first blank instead of verify().

Using times() incorrectly or omitting it causes wrong verification.

Using wrong method name in the third blank.

verify(mock, times(n)).method() checks that method was called n times on the mock or spy.