Spy objects in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Spy object in testing?

A Spy object is a special test double that wraps a real object to record how it is used, allowing verification of method calls while still executing the original methods.

intermediate

How does a Spy differ from a Mock?

A Spy calls the real methods and records interactions, while a Mock replaces the real object and only simulates behavior without calling real methods.

beginner

Show a simple example of creating a Spy in JUnit with Mockito.

Example:<br>

List<String> list = new ArrayList<>();
List<String> spyList = Mockito.spy(list);
spyList.add("test");
Mockito.verify(spyList).add("test");

intermediate

Why use a Spy object instead of a real object in tests?

Using a Spy lets you check how the object is used (like which methods were called) without losing the real behavior, helping to verify interactions and side effects.

advanced

What is a common pitfall when using Spy objects?

A common pitfall is that Spies call real methods, which can cause unwanted side effects or errors if the real methods depend on external systems or state.

What does a Spy object do in a test?

A Calls real methods and records interactions

B Only simulates methods without calling real ones

C Replaces the entire system under test

D Generates random test data

A Spy calls the real methods of the object and records how they are used for verification.

Which library is commonly used with JUnit to create Spy objects?

A Mockito

B JUnitParams

C AssertJ

D Hamcrest

Mockito is a popular mocking framework that supports creating Spy objects in JUnit tests.

What is a risk when using Spy objects in tests?

A Spy objects never call real methods

B Real methods may cause side effects

C Spy objects cannot verify method calls

D Spy objects replace the entire application

Since Spies call real methods, those methods might cause side effects or errors if not handled carefully.

How do you verify a method call on a Spy object in Mockito?

A verifyMethod(spyObject, methodName)

B spyObject.assertCalled()

C Mockito.verify(spyObject).methodName()

D spyObject.checkCall()

Mockito uses the verify() method to check if a method was called on a Spy or Mock.

Which statement is true about Spy objects?

A They do not record method calls

B They completely replace real objects with fake ones

C They cannot be used with JUnit

D They wrap real objects and allow partial mocking

Spies wrap real objects and allow you to mock some methods while keeping others real.

Explain what a Spy object is and when you would use it in testing.

Think about how a Spy lets you check real method calls.

Describe how to create and verify a Spy object in a JUnit test using Mockito.

Remember the example with a List and verify method call.