Choosing the right test double in JUnit

Introduction

Test doubles help us test parts of a program by replacing real parts with simple stand-ins. This makes testing easier and faster.

When a real part is slow or hard to use during testing.

When a real part is not ready yet but you want to test other parts.

When you want to control the behavior of a part to test specific cases.

When a real part causes side effects like sending emails or changing a database.

When you want to check if a part was used correctly during the test.

Syntax

JUnit

public class TestClass {
    @Test
    public void testMethod() {
        Dependency dep = mock(Dependency.class); // create a test double
        when(dep.someMethod()).thenReturn(someValue); // define behavior
        // use dep in the class under test
    }
}

Use mock() to create a test double that imitates a real object.

Use when(...).thenReturn(...) to set what the test double should do when called.

Examples

This creates a mock of Dependency and makes getData() return "TestData".

JUnit

Dependency dep = mock(Dependency.class);
when(dep.getData()).thenReturn("TestData");

This creates a spy that wraps a real Dependency object but overrides getData() to return "SpyData".

JUnit

Dependency dep = spy(new Dependency());
doReturn("SpyData").when(dep).getData();

This checks if someMethod() was called on the mock during the test.

JUnit

Dependency dep = mock(Dependency.class);
verify(dep).someMethod();

Sample Program

This test uses a mock as a test double for Dependency. It sets getData() to return "hello". The Service class uses this mock, and the test checks if the output is "HELLO" and if getData() was called.

JUnit

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

class Service {
    private final Dependency dep;
    public Service(Dependency dep) {
        this.dep = dep;
    }
    public String process() {
        return dep.getData().toUpperCase();
    }
}

interface Dependency {
    String getData();
}

public class ServiceTest {
    @Test
    public void testProcess() {
        Dependency dep = mock(Dependency.class);
        when(dep.getData()).thenReturn("hello");
        Service service = new Service(dep);
        String result = service.process();
        assertEquals("HELLO", result);
        verify(dep).getData();
    }
}

Important Notes

Mocks are good when you want to check interactions and control behavior.

Spies wrap real objects but let you override some behavior.

Choose the test double type based on what you want to test: behavior, state, or interaction.

Summary

Test doubles replace real parts to make testing easier and safer.

Use mocks to fake behavior and verify calls.

Pick the right test double type to match your test goal.

Practice

1. Which test double should you use in JUnit when you want to replace a real object and verify that certain methods were called during the test?

easy

A. Mock

B. Stub

C. Fake

D. Spy

Solution

1. Step 1: Understand the purpose of mocks

   Mocks are used to fake behavior and verify interactions, such as method calls.

2. Step 2: Compare with other test doubles

   Stubs provide fixed responses, fakes have working implementations, spies record calls but mocks are designed for verification.

3. Final Answer:

   Mock -> Option A

4. Quick Check:

   Verify calls = Mock [OK]

Hint: Mocks verify calls; stubs just return data [OK]

Common Mistakes:

• Confusing stubs with mocks
• Using fake when verification is needed
• Thinking spies are the same as mocks

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

easy

A. spy(new Service());

B. when(service.getData()).thenReturn("data");

C. mock(Service.class);

D. verify(service).getData();

Solution

1. Step 1: Identify stub creation syntax

   Stubs return fixed data; in Mockito, use when(...).thenReturn(...) to stub a method.

2. Step 2: Check other options

   Verify checks calls, mock creates mocks, spy wraps real objects; only when().thenReturn() stubs.

3. Final Answer:

   when(service.getData()).thenReturn("data"); -> Option B

4. Quick Check:

   Stub method = when().thenReturn() [OK]

Hint: Stubs use when().thenReturn() syntax [OK]

Common Mistakes:

• Using verify() to stub methods
• Confusing mock() with stub setup
• Using spy() instead of stub

3. Given the following JUnit test code snippet using Mockito, what will be the output of the assertion?

List<String> list = mock(List.class);
when(list.size()).thenReturn(5);
assertEquals(5, list.size());

medium

A. Test passes because size() returns 5

B. Test fails because size() returns 0 by default

C. Test throws NullPointerException

D. Test fails due to syntax error

Solution

1. Step 1: Analyze mock behavior

   The list is mocked, and size() is stubbed to return 5.

2. Step 2: Check assertion

   assertEquals(5, list.size()) compares 5 with stubbed return 5, so it passes.

3. Final Answer:

   Test passes because size() returns 5 -> Option A

4. Quick Check:

   Stubbed method returns set value [OK]

Hint: Stubbed methods return set values, so assertion passes [OK]

Common Mistakes:

• Assuming default mock returns 0
• Expecting real method call on mock
• Confusing mock with spy behavior

4. Identify the error in this JUnit test using Mockito:

@Test
void testServiceCall() {
  Service service = mock(Service.class);
  service.process();
  verify(service.process());
}

medium

A. Mock creation syntax is wrong

B. Missing stub for process() method

C. process() method cannot be mocked

D. Incorrect use of verify; should be verify(service).process();

Solution

1. Step 1: Check verify syntax

   verify() expects the mock object, then method call separately: verify(service).process();

2. Step 2: Identify error in code

   Current code calls verify(service.process()), which is incorrect syntax and causes compile error.

3. Final Answer:

   Incorrect use of verify; should be verify(service).process(); -> Option D

4. Quick Check:

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

Hint: verify() needs mock object, then method call separately [OK]

Common Mistakes:

• Calling verify with method call inside
• Forgetting to mock the object
• Confusing stub and verify usage

5. You want to test a payment processing system. The payment gateway is an external service that charges real money. Which test double should you use in your JUnit tests to avoid real charges but still verify that the payment method was called correctly?

hard

A. Use a stub to return a fixed success response

B. Use a fake with a simple in-memory implementation

C. Use a mock to verify the payment method call

D. Use a spy to wrap the real payment gateway

Solution

1. Step 1: Understand test goal

   We want to avoid real charges but verify the payment method was called.

2. Step 2: Choose test double type

   Mocks fake behavior and verify calls, perfect for checking method calls without real effects.

3. Step 3: Compare other options

   Stubs return fixed data but don't verify calls; fakes have real logic; spies call real methods, risking real charges.

4. Final Answer:

   Use a mock to verify the payment method call -> Option C

5. Quick Check:

   Verify calls without real effect = Mock [OK]

Hint: Mocks verify calls without real side effects [OK]

Common Mistakes:

• Using spy risks real external calls
• Using stub misses call verification
• Choosing fake adds unnecessary complexity