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.

Practice

1. What is the main purpose of a stub object in JUnit testing?

easy

A. To replace a real object with fixed responses for controlled testing

B. To generate random data during test execution

C. To speed up the compilation of test classes

D. To automatically fix bugs found during testing

Solution

1. Step 1: Understand the role of stub objects

   Stub objects are used to replace real parts of a system with simple implementations that return fixed data.

2. Step 2: Identify the purpose in testing

   This allows tests to run in isolation and control the test environment by providing predictable responses.

3. Final Answer:

   To replace a real object with fixed responses for controlled testing -> Option A

4. Quick Check:

   Stub objects = fixed responses for isolation [OK]

Hint: Stub = fixed answers to isolate tests quickly [OK]

Common Mistakes:

• Confusing stubs with mocks or spies
• Thinking stubs generate random data
• Believing stubs fix bugs automatically

2. Which of the following is the correct way to create a simple stub in JUnit using an anonymous class?

easy

A. MyService service = new MyService(); service.getValue() = 5;

B. MyService service = new MyService.getValue(5);

C. MyService service = new MyService() { public int getValue() { return 5; } };

D. MyService service = new MyService() -> 5;

Solution

1. Step 1: Recognize Java anonymous class syntax

   Creating a stub often uses an anonymous class overriding methods to return fixed values.

2. Step 2: Check each option's syntax

   MyService service = new MyService() { public int getValue() { return 5; } }; correctly overrides getValue() to return 5. Options A, C, and D have invalid Java syntax.

3. Final Answer:

   MyService service = new MyService() { public int getValue() { return 5; } }; -> Option C

4. Quick Check:

   Anonymous class override = MyService service = new MyService() { public int getValue() { return 5; } }; [OK]

Hint: Anonymous class overrides method with fixed return [OK]

Common Mistakes:

• Trying to assign a value directly to a method call
• Using incorrect method call syntax
• Confusing lambda syntax with anonymous classes

3. Given the stub below, what will be the output of testService.getValue()?

class TestService {
  int getValue() { return 0; }
}

TestService testService = new TestService() {
  @Override
  int getValue() { return 10; }
};
System.out.println(testService.getValue());

medium

A. Runtime exception

B. 0

C. Compilation error

D. 10

Solution

1. Step 1: Understand method overriding in anonymous class

   The anonymous class overrides getValue() to return 10 instead of 0.

2. Step 2: Determine output of the print statement

   Calling testService.getValue() invokes the overridden method, so it returns 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Overridden method returns 10 [OK]

Hint: Overridden method in stub returns new value [OK]

Common Mistakes:

• Assuming original method runs instead of overridden
• Thinking anonymous class causes compile error
• Expecting runtime exceptions without errors

4. Identify the error in the following stub creation code snippet:

MyService service = new MyService() {
  public int getValue() {
    return "five";
  }
};

medium

A. Cannot override getValue method

B. Method getValue returns wrong data type

C. Missing semicolon after class definition

D. Anonymous class syntax is incorrect

Solution

1. Step 1: Check method return type consistency

   The method getValue() is declared to return int, but returns a String "five".

2. Step 2: Verify syntax correctness

   The anonymous class syntax and semicolon are correct; overriding is allowed.

3. Final Answer:

   Method getValue returns wrong data type -> Option B

4. Quick Check:

   Return type mismatch causes error [OK]

Hint: Return type must match method signature [OK]

Common Mistakes:

• Ignoring return type mismatch
• Assuming syntax error instead of type error
• Thinking overriding is disallowed

5. You want to test a component that depends on a slow external service. Which stub implementation approach is best to speed up tests while ensuring predictable results?

hard

A. Create a stub class that returns fixed, pre-defined data instantly

B. Use the real external service but limit the number of calls

C. Mock the external service to record calls without returning data

D. Skip testing the component until the external service is faster

Solution

1. Step 1: Identify the problem with the real service

   The external service is slow, which slows down tests and makes results unpredictable.

2. Step 2: Choose a stub approach for speed and predictability

   Creating a stub that returns fixed data instantly allows fast, reliable tests without depending on the real service.

3. Step 3: Evaluate other options

   Using the real service or skipping tests delays feedback; mocking without return data doesn't help test results.

4. Final Answer:

   Create a stub class that returns fixed, pre-defined data instantly -> Option A

5. Quick Check:

   Stub fixed data = fast, predictable tests [OK]

Hint: Stub fixed data for fast, reliable tests [OK]

Common Mistakes:

• Using real slow service in tests
• Confusing mocks with stubs
• Skipping tests instead of isolating dependencies