Stub objects in JUnit - Test Execution Trace

Test Overview

This test uses a stub object to simulate a database service. It verifies that the UserService correctly returns a user name when the stub provides a fixed response.

Test Code - JUnit 5

JUnit

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

class UserService {
    private final DatabaseService databaseService;

    UserService(DatabaseService databaseService) {
        this.databaseService = databaseService;
    }

    String getUserName(int userId) {
        return databaseService.fetchUserName(userId);
    }
}

interface DatabaseService {
    String fetchUserName(int userId);
}

class DatabaseServiceStub implements DatabaseService {
    @Override
    public String fetchUserName(int userId) {
        return "Alice"; // fixed stub response
    }
}

public class UserServiceTest {
    @Test
    void testGetUserNameReturnsStubValue() {
        DatabaseService stub = new DatabaseServiceStub();
        UserService userService = new UserService(stub);

        String result = userService.getUserName(123);

        assertEquals("Alice", result);
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initializes the test class UserServiceTest	-	PASS
2	Creates DatabaseServiceStub instance	Stub object ready to return fixed user name 'Alice'	-	PASS
3	Creates UserService with stub injected	UserService uses stub instead of real database	-	PASS
4	Calls getUserName(123) on UserService	UserService calls stub.fetchUserName(123)	-	PASS
5	Stub returns fixed value 'Alice'	UserService receives 'Alice' from stub	-	PASS
6	Assert that returned user name equals 'Alice'	Comparing expected 'Alice' with actual 'Alice'	assertEquals("Alice", result)	PASS
7	Test ends successfully	Test runner reports test passed	-	PASS

Failure Scenario

Failing Condition: Stub returns a different value or null

Execution Trace Quiz - 3 Questions

What is the main purpose of the stub object in this test?

A To test the real database connection

B To simulate the database service with a fixed response

C To replace the UserService logic

D To generate random user names

Key Result

Using stub objects helps isolate the unit under test by replacing external dependencies with simple fixed responses, making tests reliable and fast.