Why different doubles serve different purposes in JUnit

Introduction

Test doubles help us replace real parts in tests to check behavior safely and easily.

When you want to check if a method was called without running real code.

When the real part is slow or not ready yet.

When you want to control what a part returns to test different cases.

When you want to isolate the part you are testing from others.

When you want to simulate errors or special situations.

Syntax

JUnit

public interface Service {
    String getData();
}

// Stub example
class StubService implements Service {
    public String getData() {
        return "stub data";
    }
}

// Mock example using Mockito
Service mockService = Mockito.mock(Service.class);
Mockito.when(mockService.getData()).thenReturn("mock data");

// Spy example using Mockito
Service realService = new RealService();
Service spyService = Mockito.spy(realService);
Mockito.when(spyService.getData()).thenReturn("spy data");

Test doubles include stubs, mocks, spies, and fakes.

Mockito is a popular library to create mocks and spies in JUnit tests.

Examples

A stub returns fixed data to help test without real logic.

JUnit

class StubService implements Service {
    public String getData() {
        return "stub data";
    }
}

A mock can verify interactions and return controlled data.

JUnit

Service mockService = Mockito.mock(Service.class);
Mockito.when(mockService.getData()).thenReturn("mock data");

A spy wraps a real object but can override some behavior.

JUnit

Service realService = new RealService();
Service spyService = Mockito.spy(realService);
Mockito.when(spyService.getData()).thenReturn("spy data");

Sample Program

This test class shows stub, mock, and spy usage with JUnit and Mockito.

JUnit

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

interface Service {
    String getData();
}

class RealService implements Service {
    public String getData() {
        return "real data";
    }
}

public class DoubleTest {

    @Test
    void testStub() {
        Service stub = new Service() {
            public String getData() {
                return "stub data";
            }
        };
        assertEquals("stub data", stub.getData());
    }

    @Test
    void testMock() {
        Service mock = Mockito.mock(Service.class);
        Mockito.when(mock.getData()).thenReturn("mock data");
        assertEquals("mock data", mock.getData());
        Mockito.verify(mock).getData();
    }

    @Test
    void testSpy() {
        RealService real = new RealService();
        Service spy = Mockito.spy(real);
        Mockito.when(spy.getData()).thenReturn("spy data");
        assertEquals("spy data", spy.getData());
        Mockito.verify(spy).getData();
    }
}

Important Notes

Use stubs to provide simple fixed responses.

Use mocks to check if methods were called and control returns.

Use spies to wrap real objects but override some behavior.

Summary

Different doubles help test parts safely and clearly.

Stubs return fixed data, mocks verify calls, spies wrap real objects.

Choosing the right double makes tests easier and more reliable.

Practice

1. Which test double is primarily used to provide predefined data to the code under test without verifying interactions?

easy

A. Stub

B. Mock

C. Spy

D. Fake

Solution

1. Step 1: Understand the role of a stub

   A stub provides fixed data or responses to the code being tested but does not check if methods were called.

2. Step 2: Differentiate from mocks and spies

   Mocks verify interactions, spies do both spying and stubbing, and fakes are simplified implementations. Only stubs focus on providing data without verification.

3. Final Answer:

   Stub -> Option A

4. Quick Check:

   Stub = provides data only [OK]

Hint: Stubs give data, mocks check calls, spies do both [OK]

Common Mistakes:

• Confusing mocks with stubs
• Thinking spies only provide data
• Mixing fakes with stubs

2. Which of the following is the correct way to create a mock object using Mockito in JUnit?

easy

A. MyClass mock = spy(new MyClass());

B. MyClass mock = new MyClass();

C. MyClass mock = Mockito.mock(MyClass.class);

D. MyClass mock = new FakeMyClass();

Solution

1. Step 1: Recall Mockito syntax for mocks

   Mockito.mock(Class.class) creates a mock object for the given class.

2. Step 2: Identify incorrect options

   MyClass mock = new MyClass(); creates a real object, not a mock. MyClass mock = spy(new MyClass()); creates a spy, not a mock. MyClass mock = new FakeMyClass(); uses a fake class, not a mock.

3. Final Answer:

   MyClass mock = Mockito.mock(MyClass.class); -> Option C

4. Quick Check:

   Mockito.mock() creates mocks [OK]

Hint: Use Mockito.mock(Class.class) to create mocks [OK]

Common Mistakes:

• Using new keyword instead of Mockito.mock
• Confusing spy() with mock()
• Assuming fake classes are mocks

3. Given the following code snippet using Mockito:

List<String> list = Mockito.spy(new ArrayList<>());
list.add("one");
Mockito.verify(list).add("one");
System.out.println(list.size());

What will be printed?

medium

A. 0

B. null

C. Throws an exception

D. 1

Solution

1. Step 1: Understand spy behavior

   A spy wraps a real object and records interactions but calls real methods unless stubbed.

2. Step 2: Analyze the code execution

   Adding "one" to the spy list calls the real add method, so the list size becomes 1. The verify confirms the add call happened.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   Spy calls real methods and records calls [OK]

Hint: Spy calls real methods and tracks calls [OK]

Common Mistakes:

• Thinking spy does not call real methods
• Assuming size remains 0 after add
• Expecting null or exception output

4. Identify the error in this Mockito test code:

MyService service = Mockito.mock(MyService.class);
service.process();
Mockito.verify(service.process());

medium

A. Missing MockitoAnnotations.initMocks(this) call

B. verify() should receive a method call without parentheses

C. verify() should be called before process()

D. process() method cannot be mocked

Solution

1. Step 1: Understand Mockito verify syntax

   Mockito.verify() expects the mock object as argument, then the method call without parentheses inside verify.

2. Step 2: Identify the mistake in code

   The code calls verify(service.process()), which calls process() immediately and passes its result. Correct is verify(service).process().

3. Final Answer:

   verify() should receive a method call without parentheses -> Option B

4. Quick Check:

   Use verify(mock).method() syntax [OK]

Hint: Use verify(mock).method() without parentheses inside verify [OK]

Common Mistakes:

• Calling method inside verify() parentheses
• Calling verify before method execution
• Confusing mock initialization with verification

5. You want to test a method that calls an external payment service. You need to check that the payment method was called with correct parameters, but also want to return a fixed success response. Which test double should you use?

hard

A. Mock

B. Spy

C. Stub

D. Fake

Solution

1. Step 1: Understand requirements for the test double

   You want to verify the method call (like a mock) and also provide a fixed response (like a stub).

2. Step 2: Identify the double that combines both features

   A mock verifies interactions and can return fixed responses when configured with when(mock.method(params)).thenReturn(success).

3. Final Answer:

   Mock -> Option A

4. Quick Check:

   Mock = verify calls + stub data [OK]

Hint: Mock verifies calls and stubs responses [OK]

Common Mistakes:

• Using stub which can't verify calls
• Using spy which executes real methods unless stubbed
• Confusing fake with mock