Why different doubles serve different purposes in JUnit - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of a Mock in Unit Testing

In JUnit testing, what is the main purpose of using a mock double?

A To simulate the behavior of real objects and verify interactions with them

B To replace the entire system under test with a fake implementation

C To store test data for later use in assertions

D To generate random input data automatically during tests

💡 Hint

Think about how mocks help check if certain methods were called.

Mocks simulate real objects and allow verification of method calls and interactions, which helps ensure the tested code behaves correctly.

🧠 Conceptual

intermediate

Difference Between Stub and Fake

Which statement best describes the difference between a stub and a fake test double?

A A stub records interactions, while a fake only returns null values

B A stub provides predefined responses, while a fake has a working implementation but is simpler than the real object

C A stub is used for integration tests, while a fake is used only in UI tests

D A stub is a real object, while a fake is a mock object

💡 Hint

Consider which double has actual logic and which just returns fixed data.

Stubs return fixed data for testing, while fakes have simple working implementations that behave like the real object but are easier to use.

❓ Predict Output

advanced

JUnit Mock Verification Output

What will be the output of the following JUnit test using Mockito when the verification fails?

JUnit

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

class ServiceTest {
    @Test
    void testServiceCall() {
        Service mockService = mock(Service.class);
        // No method call on mockService
        verify(mockService).performAction();
    }
}

interface Service {
    void performAction();
}

A Test passes with no output

B NullPointerException at verify line

C org.mockito.exceptions.verification.junit.ArgumentsAreDifferent: Wanted but not invoked: performAction()

D Compilation error due to missing method implementation

💡 Hint

Think about what happens if you verify a method call that never happened.

Mockito throws a verification error if a method expected to be called was never invoked on the mock.

❓ assertion

advanced

Correct Assertion for Stubbed Method Return

Given a stubbed method in JUnit using Mockito that returns a fixed value, which assertion correctly verifies the returned value?

JUnit

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

class CalculatorTest {
    @Test
    void testAdd() {
        Calculator calc = mock(Calculator.class);
        when(calc.add(2, 3)).thenReturn(5);
        int result = calc.add(2, 3);
        // Which assertion is correct here?
    }
}

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

A assertFalse(result == 5);

B assertTrue(result == 6);

C assertNull(result);

D assertEquals(5, result);

💡 Hint

Check the expected value from the stubbed method.

The stubbed method returns 5 for inputs 2 and 3, so the assertion must check for 5.

❓ framework

expert

Choosing the Right Test Double for Integration Testing

In a complex integration test involving a database and external API, which test double is most appropriate to isolate the external API while keeping realistic behavior for the database?

A Use a fake for the external API and real database connection

B Use a stub for the database and a mock for the external API

C Use a mock for the database and a stub for the external API

D Use a mock for both the database and external API

💡 Hint

Consider which double provides simple working behavior and which verifies interactions.

Fakes provide simple working implementations suitable for external APIs in integration tests, while using the real database keeps realistic data behavior.