Mock objects in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a mock object in unit testing?

A mock object is a fake version of a real object used in tests to simulate behavior and control interactions without using the real object.

beginner

Why do we use mock objects in tests?

We use mock objects to isolate the code being tested, avoid dependencies on external systems, and test specific behaviors easily.

beginner

In JUnit with Mockito, how do you create a mock object?

You create a mock object using Mockito.mock(ClassName.class), for example: UserService mockService = Mockito.mock(UserService.class);

intermediate

What is the difference between a mock and a stub?

A stub provides predefined responses to calls, while a mock also verifies that certain methods were called with expected arguments.

intermediate

How do you verify that a method was called on a mock object in Mockito?

Use Mockito.verify(mockObject).methodName(arguments) to check if the method was called with the expected arguments.

What is the main purpose of using mock objects in unit tests?

A To isolate the unit under test from external dependencies

B To slow down the test execution

C To replace the entire application

D To write production code

Mock objects help isolate the code being tested by simulating dependencies, making tests faster and more reliable.

Which Mockito method creates a mock object?

A Mockito.mock()

B Mockito.create()

C Mockito.fake()

D Mockito.stub()

Mockito.mock(Class.class) creates a mock object of the specified class.

How do you check if a method was called on a mock in Mockito?

A Mockito.check()

B Mockito.call()

C Mockito.assert()

D Mockito.verify()

Mockito.verify(mock).method() checks if the method was called on the mock object.

What is a stub compared to a mock?

A They are exactly the same

B A stub verifies method calls, a mock provides responses

C A stub provides predefined responses, a mock verifies method calls

D A stub is used only in integration tests

Stubs provide fixed responses; mocks also verify interactions.

Which of these is NOT a benefit of using mock objects?

A Faster test execution

B Testing real database performance

C Isolating tests from external systems

D Controlling test scenarios easily

Mocks do not test real database performance; they simulate dependencies.

Explain what a mock object is and why it is useful in unit testing.

Think about how you can test a part of your code without using the real parts it depends on.

Describe how you create and verify a mock object method call using Mockito in JUnit.

Remember the two main steps: make the fake object, then check if it was used as expected.