Dummy objects in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a dummy object for the Logger interface.

JUnit

Logger dummyLogger = [1];

Drag options to blanks, or click blank then click option'

ALogger()

Bnew Logger()

Cdummy(Logger.class)

Dmock(Logger.class)

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to instantiate an interface directly with new.

Using a non-existent method like dummy().

In JUnit with Mockito, mock(Logger.class) creates a dummy object for the Logger interface.

2fill in blank

medium

Complete the code to create a dummy List object using Mockito.

JUnit

List<String> dummyList = [1];

Drag options to blanks, or click blank then click option'

Amock(List.class)

BList()

Cdummy(List.class)

Dnew ArrayList<>()

Attempts:

3 left

💡 Hint

Common Mistakes

Using new ArrayList() which is a real object, not a dummy.

Trying to call dummy() which does not exist.

Mockito's mock(List.class) creates a dummy List object for testing.

3fill in blank

hard

Fix the error in the dummy object creation for the Service interface.

JUnit

Service dummyService = [1];

Drag options to blanks, or click blank then click option'

Amock(Service.class)

Bnew Service()

Cdummy(Service.class)

DService()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to instantiate interface with new.

Using a non-existent dummy() method.

Interfaces cannot be instantiated with new. Use Mockito's mock(Service.class) to create a dummy.

4fill in blank

hard

Fill both blanks to create a dummy object and verify it was used once.

JUnit

Repository dummyRepo = [1];
verify(dummyRepo).[2]();

Drag options to blanks, or click blank then click option'

Amock(Repository.class)

Bsave

Cdelete

Dnew Repository()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to instantiate interface with new.

Verifying a method that was not called.

Create a dummy with mock(Repository.class) and verify the save() method was called.

5fill in blank

hard

Fill all three blanks to create a dummy object, stub a method, and assert the stubbed value.

JUnit

Calculator dummyCalc = [1];
when(dummyCalc.[2](5, 3)).thenReturn([3]);
assertEquals(8, dummyCalc.add(5, 3));

Drag options to blanks, or click blank then click option'

Amock(Calculator.class)

Badd

C8

Dcalculate

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method names in stubbing.

Not stubbing the method before asserting.

Create a dummy with mock(Calculator.class), stub the add method to return 8, and assert the result.

Practice

1. What is the main purpose of a dummy object in JUnit testing?

easy

A. To perform actual logic and return real data

B. To replace the entire system under test

C. To fill method parameters when the object is not used in the test

D. To generate random test data automatically

Solution

1. Step 1: Understand dummy object role

   Dummy objects are simple placeholders used only to fill method parameters without affecting test logic.

2. Step 2: Differentiate from other test doubles

   Unlike mocks or stubs, dummy objects do not perform any behavior or return values.

3. Final Answer:

   To fill method parameters when the object is not used in the test -> Option C

4. Quick Check:

   Dummy object = placeholder parameter [OK]

Hint: Dummy objects fill unused parameters only [OK]

Common Mistakes:

• Confusing dummy objects with mocks or stubs
• Thinking dummy objects execute logic
• Using dummy objects to test behavior

2. Which of the following is the correct way to create a dummy object for a parameter of type List<String> in JUnit?

easy

A. List<String> dummy = new LinkedList<>(Arrays.asList("test"));

B. List<String> dummy = new ArrayList<>();

C. List<String> dummy = null;

D. List<String> dummy = Mockito.mock(List.class);

Solution

1. Step 1: Identify dummy object creation

   A dummy object should be a simple instance without behavior, so creating an empty ArrayList works.

2. Step 2: Evaluate other options

   Null is not a dummy object; a LinkedList with data is more than dummy; Mockito.mock creates a mock, not a dummy.

3. Final Answer:

   List<String> dummy = new ArrayList<>(); -> Option B

4. Quick Check:

   Empty simple instance = dummy object [OK]

Hint: Use simple empty instance for dummy objects [OK]

Common Mistakes:

• Using null instead of a dummy object
• Creating mocks instead of dummies
• Adding data to dummy objects unnecessarily

3. Consider the following JUnit test snippet:

public void testProcess() {
    Processor processor = new Processor();
    Data dummyData = new Data(); // dummy object
    boolean result = processor.process(dummyData);
    assertTrue(result);
}

What is the role of dummyData here?

medium

A. It acts as a dummy object to satisfy method parameter

B. It provides real data for processing

C. It mocks the Data class behavior

D. It stubs the process method output

Solution

1. Step 1: Analyze dummyData usage

   dummyData is created but not configured or used to affect logic, so it acts as a placeholder.

2. Step 2: Understand dummy vs mock/stub

   It is not mocking or stubbing behavior, just filling the parameter for process method.

3. Final Answer:

   It acts as a dummy object to satisfy method parameter -> Option A

4. Quick Check:

   Dummy object = unused parameter filler [OK]

Hint: Dummy objects fill parameters without behavior [OK]

Common Mistakes:

• Assuming dummyData affects test logic
• Confusing dummy with mock or stub
• Thinking dummyData controls output

4. Identify the error in this JUnit test using a dummy object:

@Test
public void testCalculate() {
    Calculator calc = new Calculator();
    int result = calc.calculate(null); // passing null as dummy
    assertEquals(0, result);
}

medium

A. Test method missing @Before annotation

B. The assertEquals syntax is incorrect

C. Calculator class cannot be instantiated

D. Passing null instead of a dummy object may cause NullPointerException

Solution

1. Step 1: Check dummy object usage

   Passing null is not a dummy object; it may cause NullPointerException if method uses the parameter.

2. Step 2: Verify other options

   assertEquals syntax is correct; Calculator instantiation is valid; @Before is optional here.

3. Final Answer:

   Passing null instead of a dummy object may cause NullPointerException -> Option D

4. Quick Check:

   Null ≠ dummy object [OK]

Hint: Never use null as dummy object [OK]

Common Mistakes:

• Using null instead of dummy object
• Ignoring possible NullPointerException
• Confusing test annotations

5. You have a method sendEmail(User user, EmailContent content) where content is not used in your test. How should you use a dummy object for content in your JUnit test to keep it clean and safe?

hard

A. Create an empty EmailContent instance with default constructor

B. Pass null for content parameter

C. Use a Mockito mock for EmailContent

D. Create EmailContent with real data for testing

Solution

1. Step 1: Understand dummy object purpose

   Since content is unused, a simple empty instance suffices as dummy.

2. Step 2: Avoid null and mocks for dummy

   Null risks errors; mocks add complexity; real data is unnecessary here.

3. Final Answer:

   Create an empty EmailContent instance with default constructor -> Option A

4. Quick Check:

   Empty instance = dummy object [OK]

Hint: Use simple empty instance for unused parameters [OK]

Common Mistakes:

• Passing null causing NullPointerException
• Using mocks instead of dummy objects
• Adding unnecessary real data