Mock
- Java course
- Advanced
- Lesson 49/82
- 5 min
Learn mock objects with Mockito.mock.
Mock
A mock is a fake object that mimics the behavior of a real one. With Mockito.mock() you create a mock of a class; tests are then isolated from dependencies (database, network).
UserRepository repo = mock(UserRepository.class);
when(repo.findById(1)).thenReturn(new User("Alex"));
User u = repo.findById(1);Mocks make tests fast and predictable. You can try specific scenarios without connecting to a real database. This is the foundation of unit testing; you work isolated from side effects.
Mockito setup
Mockito is used together with JUnit 5. The mock() method is statically imported; this allows short call syntax.
import static org.mockito.Mockito.*;
UserRepository repo = mock(UserRepository.class);
MailService mail = mock(MailService.class);Here repo and mail accept method calls like real objects; by default they return empty values. When you define behavior with when, they return specific values. This gives the test control over the dependencies.
Practice Mock a UserRepository
Define a User class (with a name field). Define a UserRepository interface with a User findById(int id) method. In a MockDemo class write a main method; build a real Map<Integer, User>, call put(1, new User("Alex")) and print the name of the user fetched with get(1). This simple structure simulates a mock.
Starter code
import java.util.*;
// Define the User class (name field)
// Define the UserRepository interface (findById method)
// Write main in MockDemo and print the user's nameSample solution
import java.util.*;
class User {
String name;
User(String name) { this.name = name; }
}
interface UserRepository {
User findById(int id);
}
class MockDemo {
public static void main(String[] args) {
Map<Integer, User> repo = new HashMap<>();
repo.put(1, new User("Alex"));
System.out.println(repo.get(1).name);
}
}Expected output
Alex
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does a mock object do?
- Mimics the real object's behavior Correct answer
- Connects to a real database
- Compiles the test
- Produces a test report
Explanation: A mock is a fake object that mimics the real one's behavior.
2 Complete the mock call
import static org.mockito.Mockito.*;
UserRepository repo = ____(UserRepository.class);import static org.mockito.Mockito.*;
UserRepository repo = mock(UserRepository.class);Explanation: mock() creates a fake object.
3 Match the Mockito concepts
| Term | Meaning |
|---|---|
mock |
Creates a fake object |
when |
Defines behavior |
verify |
Checks calls |
spy |
Wraps a real object |