Mocking void methods in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to mock a void method using Mockito.

JUnit

doNothing().when(mockedObject).[1]();

Drag options to blanks, or click blank then click option'

AvoidMethod()

BvoidMethod(void)

CvoidMethod

DvoidMethod{}

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses after the method name inside when(), causing syntax errors.

The correct way to specify the method call in Mockito's doNothing().when() is with parentheses to indicate the method invocation. So, voidMethod() is correct.

2fill in blank

medium

Complete the code to verify that a void method was called exactly once.

JUnit

verify(mockedObject, [1]()).voidMethod();

Drag options to blanks, or click blank then click option'

Atimes(0)

BatLeastOnce()

Cnever()

Dtimes(1)

Attempts:

3 left

💡 Hint

Common Mistakes

Using times(0) or never() when expecting the method to be called once.

To check that a method was called exactly once, use times(1) with verify().

3fill in blank

hard

Fix the error in the code to stub a void method to throw an exception.

JUnit

doThrow(new RuntimeException()).when(mockedObject).[1]();

Drag options to blanks, or click blank then click option'

AvoidMethod{}

BvoidMethod[]

CvoidMethod()

DvoidMethod

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses after the method name inside when(), causing syntax errors.

When stubbing void methods with doThrow(), the method call should include parentheses inside when().

4fill in blank

hard

Fill both blanks to stub a void method to do nothing and verify it was called twice.

JUnit

do[1]().when(mockedObject).voidMethod();
verify(mockedObject, [2]()).voidMethod();

Drag options to blanks, or click blank then click option'

ANothing

Btimes(1)

Ctimes(2)

DThrow(new RuntimeException())

Attempts:

3 left

💡 Hint

Common Mistakes

Using times(1) when expecting two calls.

Using doThrow() instead of doNothing() when no exception is expected.

doNothing() stubs the void method to do nothing, and times(2) verifies it was called twice.

5fill in blank

hard

Fill all three blanks to stub a void method to throw an exception, verify it was called once, and reset the mock.

JUnit

do[1](new RuntimeException()).when(mockedObject).voidMethod();
verify(mockedObject, [2]()).voidMethod();
[3](mockedObject);

Drag options to blanks, or click blank then click option'

ANothing

Btimes(1)

Creset

DThrow

Attempts:

3 left

💡 Hint

Common Mistakes

Using doNothing() instead of doThrow() when expecting an exception.

Forgetting to reset the mock after verification.

doThrow() stubs the void method to throw an exception, times(1) verifies it was called once, and reset() clears the mock's interactions.

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