Mocking void methods in JUnit

Introduction

Sometimes, methods do not return any value but still perform important actions. Mocking void methods helps us test code that depends on these actions without running the real method.

When a method changes data or state but returns nothing

When a method sends messages or logs information

When a method calls external systems but you want to avoid real calls

When you want to check if a void method was called correctly

When you want to simulate exceptions thrown by void methods

Syntax

JUnit

doNothing().when(mockObject).voidMethod();
doThrow(new Exception()).when(mockObject).voidMethod();

Use doNothing() to skip the real void method.

Use doThrow() to simulate exceptions from void methods.

Examples

This tells the mock list to do nothing when clear() is called.

JUnit

doNothing().when(mockList).clear();

This makes the mock service throw an exception when process() is called.

JUnit

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

Sample Program

This test mocks a List object. It makes clear() do nothing and verifies it was called once. It also makes add("test") throw an exception and prints the exception message.

JUnit

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;
import java.util.List;

public class VoidMethodMockTest {

    @Test
    void testVoidMethodMocking() {
        List<String> mockList = mock(List.class);

        // Mock clear() to do nothing
        doNothing().when(mockList).clear();

        // Call clear()
        mockList.clear();

        // Verify clear() was called once
        verify(mockList, times(1)).clear();

        // Mock add() to throw exception
        doThrow(new RuntimeException("Add failed")).when(mockList).add("test");

        try {
            mockList.add("test");
        } catch (RuntimeException e) {
            System.out.println(e.getMessage());
        }
    }
}

Important Notes

Always verify that the void method was called if that matters for your test.

Use doAnswer() if you want to run custom code when the void method is called.

Summary

Void methods do not return values but can be mocked using doNothing() or doThrow().

Mocking void methods helps test code without running real side effects.

Always verify calls to void methods if your test depends on them.

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