Stub objects in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple stub method call

What will be the output of this JUnit test using a stub object?

JUnit

public interface Calculator {
    int add(int a, int b);
}

public class CalculatorStub implements Calculator {
    @Override
    public int add(int a, int b) {
        return 42; // stub always returns 42
    }
}

@Test
public void testAdd() {
    Calculator calc = new CalculatorStub();
    int result = calc.add(5, 3);
    System.out.println(result);
}

A 42

B 8

C 0

D Compilation error

💡 Hint

Remember, a stub returns fixed values regardless of input.

The stub method add always returns 42, ignoring the input values. So the printed output is 42.

❓ assertion

intermediate

Correct assertion for stubbed method

Which assertion correctly verifies the stubbed method returns the expected value?

JUnit

public interface Service {
    String getData();
}

public class ServiceStub implements Service {
    @Override
    public String getData() {
        return "stubbed data";
    }
}

@Test
public void testGetData() {
    Service service = new ServiceStub();
    String data = service.getData();
    // Which assertion is correct here?
}

A assertFalse(data.equals("stubbed data"));

B assertEquals("stubbed data", data);

C assertNull(data);

D assertTrue(data == "stubbed data");

💡 Hint

Use the proper JUnit assertion to compare string values.

assertEquals compares string content correctly. Using == compares references and may fail.

🔧 Debug

advanced

Identify the stub causing test failure

Given this test and stub, why does the test fail?

JUnit

public interface UserRepository {
    boolean exists(String username);
}

public class UserRepositoryStub implements UserRepository {
    @Override
    public boolean exists(String username) {
        return false; // stub always returns false
    }
}

@Test
public void testUserExists() {
    UserRepository repo = new UserRepositoryStub();
    boolean exists = repo.exists("admin");
    assertTrue(exists);
}

A The test is missing a setup method to initialize the stub.

B The stub method signature is incorrect causing a compile error.

C The stub always returns false, so assertTrue fails.

D The assertTrue method is used incorrectly with a boolean.

💡 Hint

Check what the stub method returns and what the assertion expects.

The stub always returns false, but the test expects true, so the assertion fails.

🧠 Conceptual

advanced

Purpose of stub objects in unit testing

What is the main purpose of using stub objects in unit testing?

A To replace real components with fixed responses to isolate the unit under test.

B To automatically generate test cases based on code coverage.

C To measure performance and load of the application.

D To simulate user input events in UI testing.

💡 Hint

Think about how stubs help focus tests on one part of the code.

Stubs provide fixed responses to isolate the tested unit from dependencies.

❓ framework

expert

Using Mockito to create a stub method

Which Mockito code snippet correctly stubs a method to return "hello" when called?

JUnit

public interface GreetingService {
    String greet();
}

@Test
public void testGreet() {
    GreetingService mockService = Mockito.mock(GreetingService.class);
    // Which line stubs greet() to return "hello"?
}

A Mockito.mockReturn(mockService.greet(), "hello");

B Mockito.stub(mockService.greet()).toReturn("hello");

C Mockito.given(mockService.greet()).willReturn("hello");

D Mockito.when(mockService.greet()).thenReturn("hello");

💡 Hint

Check the correct Mockito syntax for stubbing methods.

Mockito.when(...).thenReturn(...) is the correct syntax to stub a method.