Mocking void methods in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a void method in Java?

A void method is a method that does not return any value. It performs an action but returns nothing.

beginner

Why do we need to mock void methods in unit testing?

We mock void methods to control their behavior during tests, especially if they cause side effects or interact with external systems, so tests can run reliably and independently.

intermediate

How do you mock a void method using Mockito in JUnit?

Use Mockito's doNothing(), doThrow(), or doAnswer() methods with when() to define behavior for void methods. For example:
doNothing().when(mockObject).voidMethod();

intermediate

What does the following code do?

doThrow(new RuntimeException()).when(mockService).voidMethod();

It tells Mockito to throw a RuntimeException when the voidMethod() is called on mockService during the test.

intermediate

Explain the difference between when() and doNothing() when mocking void methods.

when() cannot be used directly with void methods because they return nothing. Instead, doNothing() is used to specify that the void method should do nothing when called during the test.

Which Mockito method is used to mock a void method that should do nothing when called?

A doNothing()

B when()

C thenReturn()

D verify()

doNothing() is used to mock void methods to do nothing when called.

What happens if you try to use when() directly on a void method in Mockito?

A It causes a compilation error

B It throws an exception at runtime

C It works normally

D It silently ignores the call

Using when() directly on void methods causes a compilation error because void methods do not return values.

How can you make a mocked void method throw an exception when called?

A when(mock.voidMethod()).thenThrow(new Exception());

B doNothing().when(mock).voidMethod();

C doThrow(new Exception()).when(mock).voidMethod();

D verify(mock).voidMethod();

doThrow() is used with when() to specify that a void method should throw an exception.

What is the purpose of mocking void methods in unit tests?

A To test the return value

B To increase code complexity

C To avoid writing tests

D To control side effects and interactions

Mocking void methods helps control side effects and external interactions during tests.

Which of the following is NOT a valid way to mock a void method in Mockito?

A doNothing()

B thenReturn()

C doThrow()

D doAnswer()

thenReturn() cannot be used with void methods because they do not return values.

Describe how to mock a void method in JUnit using Mockito and explain why this is useful.

Think about how void methods behave and how Mockito handles them differently.

Explain the difference between mocking a void method and a method that returns a value.

Focus on how Mockito syntax changes based on method return type.

Practice

1. What is the main purpose of mocking void methods in JUnit tests?

easy

A. To convert void methods into methods that return values

B. To control their behavior and verify interactions without executing actual code

C. To automatically generate return values for void methods

D. To disable all tests that call void methods

Solution

1. Step 1: Understand void methods in tests

   Void methods do not return values, so mocking helps control their side effects during tests.

2. Step 2: Purpose of mocking void methods

   Mocking allows skipping actual execution and verifying if the method was called, without running real code.

3. Final Answer:

   To control their behavior and verify interactions without executing actual code -> Option B

4. Quick Check:

   Mock void methods = control behavior + verify calls [OK]

Hint: Mock void methods to control calls without running code [OK]

Common Mistakes:

• Thinking void methods return values
• Assuming mocking changes method return type
• Believing mocking disables all tests

2. Which Mockito syntax correctly mocks a void method sendEmail() to do nothing when called?

easy

A. doNothing().when(mock).sendEmail();

B. verify(mock).doNothing().sendEmail();

C. mock.sendEmail().doNothing();

D. when(mock.sendEmail()).thenReturn(null);

Solution

1. Step 1: Recall Mockito syntax for void methods

   Mockito uses doNothing().when(mock).method() to mock void methods doing nothing.

2. Step 2: Analyze options

   doNothing().when(mock).sendEmail(); matches correct syntax; others misuse when() or verify().

3. Final Answer:

   doNothing().when(mock).sendEmail(); -> Option A

4. Quick Check:

   Mock void method to do nothing = doNothing().when() [OK]

Hint: Use doNothing().when(mock).voidMethod() to mock void methods [OK]

Common Mistakes:

• Using when() with void methods incorrectly
• Trying to chain doNothing() after method call
• Confusing verify() with mocking syntax

3. Given the code snippet:

doThrow(new RuntimeException()).when(mock).deleteFile();
mock.deleteFile();

What happens when mock.deleteFile() is called?

medium

A. The test fails to compile

B. The method executes normally without exception

C. The method returns null

D. A RuntimeException is thrown

Solution

1. Step 1: Understand doThrow() usage

   doThrow() tells Mockito to throw the specified exception when the void method is called.

2. Step 2: Analyze method call effect

   Calling mock.deleteFile() triggers the RuntimeException as mocked.

3. Final Answer:

   A RuntimeException is thrown -> Option D

4. Quick Check:

   doThrow() causes exception on void method call [OK]

Hint: doThrow() makes void method throw exception when called [OK]

Common Mistakes:

• Expecting method to run normally
• Thinking void methods return null
• Confusing doThrow() with doNothing()

4. Identify the error in this Mockito code snippet:

doNothing().when(mock.deleteFile());

medium

A. Incorrect use of when() with method call inside

B. Missing exception type in doNothing()

C. doNothing() cannot be used with void methods

D. No error, code is correct

Solution

1. Step 1: Check Mockito syntax for mocking void methods

   Correct syntax is doNothing().when(mock).method(), not passing method call inside when().

2. Step 2: Identify error in code

   Code incorrectly calls method inside when(), causing syntax error.

3. Final Answer:

   Incorrect use of when() with method call inside -> Option A

4. Quick Check:

   when() expects mock object, not method call [OK]

Hint: Use when(mock).method(), not when(method()) [OK]

Common Mistakes:

• Calling method inside when() instead of after when(mock)
• Thinking doNothing() needs exception type
• Believing doNothing() can't mock void methods

5. You want to verify that a void method logAction() was called exactly twice during a test. Which Mockito code correctly does this?

hard

A. verify(mock).logAction(times(2));

B. doNothing().when(mock).logAction(); verify(mock).logAction();

C. verify(mock, times(2)).logAction();

D. when(mock.logAction()).thenReturn(2);

Solution

1. Step 1: Recall Mockito verify syntax for call count

   Use verify(mock, times(n)).method() to check method called n times.

2. Step 2: Analyze options

   verify(mock, times(2)).logAction(); uses correct syntax; others misuse verify or when with void methods.

3. Final Answer:

   verify(mock, times(2)).logAction(); -> Option C

4. Quick Check:

   Verify call count = verify(mock, times(n)) [OK]

Hint: Use verify(mock, times(n)) to check void method call count [OK]

Common Mistakes:

• Using when() with void methods to check calls
• Misplacing times() inside method call
• Not specifying times() for multiple calls