Dummy objects in JUnit - Build an Automation Script

Use dummy objects to satisfy method parameters in unit test

Preconditions (2)

Step 1: Create a dummy object to pass as a parameter to a method that requires an object but does not use it

Step 2: Write a unit test for Calculator.add method that requires a dummy object parameter

Step 3: Call the add method with two integers and the dummy object

Step 4: Verify the add method returns the correct sum

✅ Expected Result: The test passes successfully, confirming the add method works and the dummy object satisfies the parameter requirement without affecting the test

Automation Requirements - JUnit 5

Assertions Needed:

Assert that the add method returns the correct sum

Best Practices:

Use dummy objects only to fill parameters not used in the test

Keep dummy objects simple and minimal

Use @Test annotation for test methods

Use assertions from org.junit.jupiter.api.Assertions

Automated Solution

JUnit

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

class Calculator {
    // Method that requires a dummy object parameter but does not use it
    public int add(int a, int b, Object dummy) {
        return a + b;
    }
}

public class CalculatorTest {

    // Dummy object class
    static class DummyObject {}

    @Test
    void testAddWithDummyObject() {
        Calculator calculator = new Calculator();
        DummyObject dummy = new DummyObject(); // dummy object to satisfy parameter

        int result = calculator.add(5, 7, dummy);

        assertEquals(12, result, "The add method should return the sum of two numbers");
    }
}

This test shows how to use a dummy object to satisfy a method parameter that is not used in the logic. The Calculator.add method requires three parameters, but the third parameter is not used. We create a simple DummyObject class and pass an instance to the method. The test asserts that the sum is correct, ignoring the dummy object. This keeps the test focused and simple.

Common Mistakes - 3 Pitfalls

Using a real object with complex behavior instead of a simple dummy

Not using @Test annotation on the test method

Asserting on the dummy object instead of the actual method result

Bonus Challenge

Now add data-driven testing with 3 different pairs of integers to test the add method with dummy objects

Show Hint

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