when().thenReturn() stubbing in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does when().thenReturn() do in JUnit testing?

when().thenReturn() is used to tell a mock object what value to return when a specific method is called. It helps simulate behavior for testing.

beginner

How do you stub a method getName() of a mock object userMock to return "Alice"?

when(userMock.getName()).thenReturn("Alice");

beginner

Why is stubbing useful in unit tests?

Stubbing lets you control the output of dependencies so you can test your code in isolation without relying on real objects or external systems.

intermediate

What happens if you call a method on a mock without stubbing it first?

By default, the mock returns default values like null, 0, or false. It does not execute real code.

intermediate

Can thenReturn() be used to return different values on consecutive calls?

Yes, you can chain thenReturn() calls to return different values each time the method is called.

when(mock.method()).thenReturn(1, 2);

What is the purpose of when().thenReturn() in JUnit?

A To stub a method call on a mock object

B To verify a method was called

C To create a real object

D To throw an exception

when().thenReturn() is used to stub methods on mocks, telling them what to return when called.

Which of these is a correct stubbing syntax?

A thenReturn(value).when(mock.method());

B mock.when(method()).thenReturn(value);

C when(mock.method()).thenReturn(value);

D mock.method().when().thenReturn(value);

The correct syntax is when(mock.method()).thenReturn(value);

If you do not stub a method on a mock, what will it return by default?

A The real method's return value

B A default value like null or 0

C An exception

D Nothing, it will crash

Mocks return default values like null or 0 if not stubbed.

How can you make a stub return different values on consecutive calls?

A Chain multiple <code>thenReturn()</code> calls

B Use multiple <code>when()</code> calls

C Use <code>thenThrow()</code>

D It is not possible

You can chain thenReturn() to return different values each call.

Which library provides when().thenReturn() for mocking in JUnit tests?

A TestNG

B JUnit

C Selenium

D Mockito

when().thenReturn() is part of Mockito, a mocking framework used with JUnit.

Explain how when().thenReturn() helps in unit testing with mocks.

Think about how you tell a mock what to do when a method is called.

Describe the default behavior of a mock object when a method is called without stubbing.

What happens if you forget to stub a method on a mock?