Mocking void methods in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of mocking a void method with doNothing()

What will be the output of the following JUnit test using Mockito when the void method is mocked with doNothing()?

JUnit

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

class Service {
    void performAction() {
        System.out.println("Action performed");
    }
}

public class ServiceTest {
    @Test
    void testPerformAction() {
        Service mockService = mock(Service.class);
        doNothing().when(mockService).performAction();
        mockService.performAction();
        System.out.println("Test completed");
    }
}

A Action performed\nTest completed

B No output

C NullPointerException

D Test completed

💡 Hint

Think about what doNothing() does when mocking void methods.

The doNothing() stub tells Mockito to do nothing when the void method is called, so the original method body is not executed. Therefore, "Action performed" is not printed, only "Test completed" is printed.

❓ assertion

intermediate

Correct assertion to verify a void method call

Which assertion correctly verifies that a void method performAction() was called exactly once on a mock object in JUnit with Mockito?

JUnit

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

class Service {
    void performAction() {}
}

public class ServiceTest {
    @Test
    void testVerifyCall() {
        Service mockService = mock(Service.class);
        mockService.performAction();
        // Which assertion is correct here?
    }
}

A verify(mockService).performAction(1);

B assertTrue(mockService.performAction());

C verify(mockService, times(1)).performAction();

D assertEquals(1, mockService.performAction());

💡 Hint

Mockito uses verify() to check method calls on mocks.

The verify() method with times(1) checks that performAction() was called exactly once. The other options are invalid because performAction() returns void and cannot be used in assertions like assertEquals or assertTrue, and verify() does not take arguments like performAction(1).

🔧 Debug

advanced

Debugging a test with doThrow on a void method

Given the following test, what error will be thrown when mockService.performAction() is called?

JUnit

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

class Service {
    void performAction() {}
}

public class ServiceTest {
    @Test
    void testException() {
        Service mockService = mock(Service.class);
        doThrow(new RuntimeException("Error occurred")).when(mockService).performAction();
        mockService.performAction();
    }
}

A NullPointerException

B RuntimeException with message "Error occurred"

C MockitoException

D No exception, method does nothing

💡 Hint

doThrow() makes the mocked void method throw an exception when called.

The doThrow() stub causes the mocked void method to throw the specified RuntimeException when called. So the test will throw RuntimeException with the message "Error occurred".

🧠 Conceptual

advanced

Why use doAnswer() with void methods in Mockito?

Why would a tester use doAnswer() when mocking a void method instead of doNothing() or doThrow()?

A To execute custom code or logic when the void method is called

B To automatically call the real method implementation

C To ignore the method call silently without any effect

D To verify the method was never called

💡 Hint

doAnswer() lets you define what happens when the method is called.

doAnswer() allows the tester to run custom code or logic when the void method is called, such as modifying arguments or triggering side effects. doNothing() just skips the method, and doThrow() throws an exception. doAnswer() is useful for complex behaviors.

❓ framework

expert

Correct Mockito syntax to mock a void method throwing checked exception

Which option correctly mocks a void method performAction() to throw a checked exception IOException when called, using Mockito in JUnit?

JUnit

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;
import java.io.IOException;

class Service {
    void performAction() throws IOException {}
}

public class ServiceTest {
    @Test
    void testCheckedException() throws IOException {
        Service mockService = mock(Service.class);
        // Which option correctly mocks performAction() to throw IOException?
    }
}

A doThrow(new IOException("IO error")).when(mockService).performAction();

B when(mockService.performAction()).thenThrow(new IOException("IO error"));

C doThrow(IOException.class).when(mockService).performAction();

D when(mockService.performAction()).thenReturn(new IOException("IO error"));

💡 Hint

Use doThrow() for void methods throwing checked exceptions.

For void methods that throw checked exceptions, Mockito requires doThrow().when(mock).method() syntax. Option A causes a compile error because performAction() returns void and cannot be used in when(). Option A is invalid syntax. Option A tries to return an exception object which is invalid for void methods.