Fake objects in JUnit - Test Execution Trace

Test Overview

This test uses a fake object to simulate a database service. It verifies that the service returns the expected user name for a given user ID.

Test Code - JUnit

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

class UserService {
    private final Database database;

    UserService(Database database) {
        this.database = database;
    }

    String getUserName(int userId) {
        return database.getUserNameById(userId);
    }
}

interface Database {
    String getUserNameById(int userId);
}

class FakeDatabase implements Database {
    @Override
    public String getUserNameById(int userId) {
        if (userId == 1) {
            return "Alice";
        } else {
            return "Unknown";
        }
    }
}

public class UserServiceTest {

    @Test
    void testGetUserNameReturnsCorrectName() {
        Database fakeDb = new FakeDatabase();
        UserService userService = new UserService(fakeDb);

        String result = userService.getUserName(1);

        assertEquals("Alice", result);
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner is ready to execute the test method	-	PASS
2	Creates a FakeDatabase object	FakeDatabase instance is created with hardcoded user data	-	PASS
3	Creates UserService with the fake database	UserService instance holds reference to FakeDatabase	-	PASS
4	Calls getUserName(1) on UserService	UserService calls FakeDatabase.getUserNameById(1)	-	PASS
5	FakeDatabase returns "Alice" for userId 1	UserService receives "Alice"	-	PASS
6	Assert that returned name equals "Alice"	JUnit compares expected "Alice" with actual "Alice"	assertEquals("Alice", result)	PASS
7	Test ends successfully	Test method completes without errors	-	PASS

Failure Scenario

Failing Condition: FakeDatabase returns a different name or null for userId 1

Execution Trace Quiz - 3 Questions

What is the purpose of the FakeDatabase in this test?

A To connect to the real database and fetch data

B To simulate the database behavior without using a real database

C To test the database connection speed

D To replace the UserService class

Key Result

Using fake objects helps isolate the unit under test by replacing real dependencies with simple, controlled implementations. This makes tests faster and more reliable.