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?

Practice

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

easy

A. To specify a fake return value for a mocked method call

B. To execute the real method instead of a mock

C. To verify that a method was called

D. To create a new instance of the class under test

Solution

1. Step 1: Understand mocking in tests

   Mocking replaces real objects with fake ones to control behavior during tests.

2. Step 2: Role of when().thenReturn()

   This syntax sets a fake return value for a method call on a mock object.

3. Final Answer:

   To specify a fake return value for a mocked method call -> Option A

4. Quick Check:

   when().thenReturn() = fake return value [OK]

Hint: Remember: when().thenReturn() fakes method results [OK]

Common Mistakes:

• Thinking it calls the real method
• Using it on real objects instead of mocks
• Confusing it with verification methods

2. Which of the following is the correct syntax to stub a method getName() of a mock object userMock to return "Alice"?

easy

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

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

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

D. when(userMock.getName() == "Alice");

Solution

1. Step 1: Recall correct when().thenReturn() syntax

   The correct order is when(methodCall).thenReturn(value);

2. Step 2: Check each option

   when(userMock.getName()).thenReturn("Alice"); matches the correct syntax exactly; others are invalid or reversed.

3. Final Answer:

   when(userMock.getName()).thenReturn("Alice"); -> Option B

4. Quick Check:

   Correct syntax = when().thenReturn() [OK]

Hint: Syntax is when(methodCall).thenReturn(value) [OK]

Common Mistakes:

• Reversing when() and thenReturn() order
• Calling when() on the mock object instead of method
• Using equality inside when()

3. Given the code below, what will result print?

List<String> mockList = mock(List.class);
when(mockList.size()).thenReturn(5);
int result = mockList.size();
System.out.println(result);

medium

A. 0

B. null

C. 5

D. Throws NullPointerException

Solution

1. Step 1: Understand stubbing of mockList.size()

   The size() method is stubbed to return 5 whenever called on mockList.

2. Step 2: Check the printed result

   Calling mockList.size() returns 5 as stubbed, so result is 5 and prints 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Stubbed method returns set value [OK]

Hint: Stubbed methods return thenReturn() value, not default [OK]

Common Mistakes:

• Assuming default mock return 0 instead of stubbed 5
• Expecting real method execution
• Confusing null with int return

4. Identify the error in the following stubbing code:

MyService service = new MyService();
when(service.getData()).thenReturn("Mocked Data");

medium

A. Cannot stub a method on a real object, must mock it first

B. Missing semicolon after thenReturn()

C. Incorrect method name getData()

D. thenReturn() cannot return a String

Solution

1. Step 1: Check object type used in when()

   The object service is a real instance, not a mock, so stubbing is invalid.

2. Step 2: Understand mocking requirement

   Stubbing works only on mock objects created by mock() or similar methods.

3. Final Answer:

   Cannot stub a method on a real object, must mock it first -> Option A

4. Quick Check:

   Stub only mocks, not real objects [OK]

Hint: Stub only mock objects, not real instances [OK]

Common Mistakes:

• Trying to stub real objects
• Ignoring need to mock before stubbing
• Assuming thenReturn() syntax error

5. You want to stub a method calculate(int x) of a mock calculatorMock to return x * 2 for any integer input. Which is the correct way to do this using when().thenReturn()?

hard

A. when(calculatorMock.calculate(2)).thenReturn(4);

B. when(calculatorMock.calculate(anyInt())).thenReturn(x * 2);

C. when(calculatorMock.calculate(anyInt())).thenReturn(2);

D. when(calculatorMock.calculate(anyInt())).thenAnswer(invocation -> (int) invocation.getArgument(0) * 2);

Solution

1. Step 1: Understand when().thenReturn() limitation

   thenReturn() returns a fixed value, cannot compute based on input parameter.

2. Step 2: Use thenAnswer() for dynamic return

   thenAnswer() allows lambda to compute return based on method arguments.

3. Final Answer:

   when(calculatorMock.calculate(anyInt())).thenAnswer(invocation -> (int) invocation.getArgument(0) * 2); -> Option D

4. Quick Check:

   Dynamic return needs thenAnswer(), not thenReturn() [OK]

Hint: Use thenAnswer() for dynamic returns, not thenReturn() [OK]

Common Mistakes:

• Trying to compute dynamic values with thenReturn()
• Using undefined variable x in thenReturn()
• Stubbing only one input value instead of anyInt()