Dummy objects in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a test using a dummy object in JUnit

Consider the following JUnit test code using a dummy object. What will be the test execution result?

JUnit

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

class Service {
    private final Repository repo;
    Service(Repository repo) { this.repo = repo; }
    boolean isAvailable() { return repo != null; }
}

interface Repository {}

class DummyRepository implements Repository {}

public class DummyTest {
    @Test
    void testServiceWithDummy() {
        Repository dummy = new DummyRepository();
        Service service = new Service(dummy);
        assertTrue(service.isAvailable());
    }
}

A Test passes because dummy object satisfies the dependency.

B Test fails because DummyRepository does not implement any methods.

C Test throws NullPointerException at runtime.

D Test fails due to compilation error: missing method implementation.

💡 Hint

Dummy objects provide minimal implementation just to satisfy dependencies.

The dummy object DummyRepository implements the Repository interface but does not provide any methods. Since Service only checks if repo is not null, the test passes.

❓ assertion

intermediate

Correct assertion for verifying dummy object usage

You have a dummy object passed to a method that does not use it. Which assertion correctly verifies the method completes without errors?

JUnit

class Processor {
    void process(Object dummy) { /* does nothing with dummy */ }
}

Processor processor = new Processor();
Object dummy = new Object();

// Which assertion below is correct to test process(dummy)?

A assertEquals(dummy, processor.process(dummy));

B assertDoesNotThrow(() -> processor.process(dummy));

C assertNull(processor.process(dummy));

D assertThrows(Exception.class, () -> processor.process(dummy));

💡 Hint

The method returns void and does nothing, so check it does not throw exceptions.

Since process returns void and does nothing, the correct assertion is to verify it does not throw any exceptions.

🔧 Debug

advanced

Identify the problem with dummy object usage in this test

This JUnit test uses a dummy object but does not compile. What is the cause?

JUnit

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

interface Logger {
    void log(String message);
}

class Service {
    private final Logger logger;
    Service(Logger logger) { this.logger = logger; }
    void run() { logger.log("Running"); }
}

class DummyLogger implements Logger {}

public class DummyBugTest {
    @Test
    void testRun() {
        Logger dummy = new DummyLogger();
        Service service = new Service(dummy);
        service.run();
        assertTrue(true);
    }
}

A Compilation error because DummyLogger is missing interface implementation.

B Service constructor throws NullPointerException due to dummy being null.

C DummyLogger does not implement log method, causing AbstractMethodError at runtime.

D Test fails assertion because assertTrue(true) is false.

💡 Hint

Check if DummyLogger properly implements all interface methods.

DummyLogger does not provide an implementation for log, causing a compilation error as it must implement all abstract methods from the interface.

🧠 Conceptual

advanced

Purpose of dummy objects in unit testing

Which statement best describes the purpose of dummy objects in unit testing?

A They generate random data inputs to test edge cases automatically.

B They simulate complex behavior to test interactions between components.

C They replace real objects to verify method calls and returned values.

D They provide minimal implementations to satisfy method parameters without affecting test behavior.

💡 Hint

Think about why you would pass an object that is not used in the test.

Dummy objects are simple placeholders passed to satisfy method signatures but do not affect the test outcome.

❓ framework

expert

Using Mockito to create a dummy object in JUnit

Which Mockito code snippet correctly creates a dummy object for interface ServiceClient to pass into a test?

JUnit

interface ServiceClient {
    void send(String data);
}

// Choose the correct Mockito code to create a dummy ServiceClient

A ServiceClient dummy = Mockito.spy(ServiceClient.class);

B ServiceClient dummy = new ServiceClient() {};

C ServiceClient dummy = Mockito.mock(ServiceClient.class);

D ServiceClient dummy = Mockito.create(ServiceClient.class);

💡 Hint

Mockito.mock creates a dummy object for interfaces.

Mockito.mock(ServiceClient.class) creates a dummy object implementing ServiceClient with no behavior.