Stub objects in JUnit - Build an Automation Script

Verify calculation method using a stub object

Preconditions (2)

Step 1: Create a stub object that returns a fixed multiplier value of 5

Step 2: Inject the stub object into the Calculator class

Step 3: Call the calculate method with input value 10

Step 4: Observe the returned result

✅ Expected Result: The calculate method returns 50 (10 multiplied by 5 from the stub)

Automation Requirements - JUnit 5

Assertions Needed:

Assert that the calculate method returns 50 when input is 10

Best Practices:

Use stub objects to isolate the unit under test

Keep stub implementation simple and focused

Use dependency injection to provide the stub

Use clear and descriptive test method names

Automated Solution

JUnit

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

interface MultiplierService {
    int getMultiplier();
}

class Calculator {
    private final MultiplierService multiplierService;

    public Calculator(MultiplierService multiplierService) {
        this.multiplierService = multiplierService;
    }

    public int calculate(int input) {
        return input * multiplierService.getMultiplier();
    }
}

public class CalculatorTest {

    // Stub object that returns fixed multiplier 5
    static class MultiplierServiceStub implements MultiplierService {
        @Override
        public int getMultiplier() {
            return 5;
        }
    }

    @Test
    void calculate_returnsCorrectResult_usingStub() {
        MultiplierService stub = new MultiplierServiceStub();
        Calculator calculator = new Calculator(stub);

        int result = calculator.calculate(10);

        assertEquals(50, result, "Calculator should return input multiplied by stub multiplier");
    }
}

This test uses a stub object MultiplierServiceStub that always returns 5 as the multiplier. We inject this stub into the Calculator class to isolate the test from any real external service.

The test method calculate_returnsCorrectResult_usingStub calls calculate(10) and asserts the result is 50, which is 10 multiplied by the stub's fixed multiplier 5.

This approach ensures the test is fast, reliable, and focused only on the Calculator logic, not on external dependencies.

Common Mistakes - 3 Pitfalls

Not using dependency injection and creating the stub inside the class under test

Making the stub object too complex or replicating full service logic

Not asserting the expected result or using vague assertions

Bonus Challenge

Now add data-driven testing with 3 different input values and expected results using the stub

Show Hint

Practice

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

easy

A. To replace a real object with fixed responses for controlled testing

B. To generate random data during test execution

C. To speed up the compilation of test classes

D. To automatically fix bugs found during testing

Solution

1. Step 1: Understand the role of stub objects

   Stub objects are used to replace real parts of a system with simple implementations that return fixed data.

2. Step 2: Identify the purpose in testing

   This allows tests to run in isolation and control the test environment by providing predictable responses.

3. Final Answer:

   To replace a real object with fixed responses for controlled testing -> Option A

4. Quick Check:

   Stub objects = fixed responses for isolation [OK]

Hint: Stub = fixed answers to isolate tests quickly [OK]

Common Mistakes:

• Confusing stubs with mocks or spies
• Thinking stubs generate random data
• Believing stubs fix bugs automatically

2. Which of the following is the correct way to create a simple stub in JUnit using an anonymous class?

easy

A. MyService service = new MyService(); service.getValue() = 5;

B. MyService service = new MyService.getValue(5);

C. MyService service = new MyService() { public int getValue() { return 5; } };

D. MyService service = new MyService() -> 5;

Solution

1. Step 1: Recognize Java anonymous class syntax

   Creating a stub often uses an anonymous class overriding methods to return fixed values.

2. Step 2: Check each option's syntax

   MyService service = new MyService() { public int getValue() { return 5; } }; correctly overrides getValue() to return 5. Options A, C, and D have invalid Java syntax.

3. Final Answer:

   MyService service = new MyService() { public int getValue() { return 5; } }; -> Option C

4. Quick Check:

   Anonymous class override = MyService service = new MyService() { public int getValue() { return 5; } }; [OK]

Hint: Anonymous class overrides method with fixed return [OK]

Common Mistakes:

• Trying to assign a value directly to a method call
• Using incorrect method call syntax
• Confusing lambda syntax with anonymous classes

3. Given the stub below, what will be the output of testService.getValue()?

class TestService {
  int getValue() { return 0; }
}

TestService testService = new TestService() {
  @Override
  int getValue() { return 10; }
};
System.out.println(testService.getValue());

medium

A. Runtime exception

B. 0

C. Compilation error

D. 10

Solution

1. Step 1: Understand method overriding in anonymous class

   The anonymous class overrides getValue() to return 10 instead of 0.

2. Step 2: Determine output of the print statement

   Calling testService.getValue() invokes the overridden method, so it returns 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Overridden method returns 10 [OK]

Hint: Overridden method in stub returns new value [OK]

Common Mistakes:

• Assuming original method runs instead of overridden
• Thinking anonymous class causes compile error
• Expecting runtime exceptions without errors

4. Identify the error in the following stub creation code snippet:

MyService service = new MyService() {
  public int getValue() {
    return "five";
  }
};

medium

A. Cannot override getValue method

B. Method getValue returns wrong data type

C. Missing semicolon after class definition

D. Anonymous class syntax is incorrect

Solution

1. Step 1: Check method return type consistency

   The method getValue() is declared to return int, but returns a String "five".

2. Step 2: Verify syntax correctness

   The anonymous class syntax and semicolon are correct; overriding is allowed.

3. Final Answer:

   Method getValue returns wrong data type -> Option B

4. Quick Check:

   Return type mismatch causes error [OK]

Hint: Return type must match method signature [OK]

Common Mistakes:

• Ignoring return type mismatch
• Assuming syntax error instead of type error
• Thinking overriding is disallowed

5. You want to test a component that depends on a slow external service. Which stub implementation approach is best to speed up tests while ensuring predictable results?

hard

A. Create a stub class that returns fixed, pre-defined data instantly

B. Use the real external service but limit the number of calls

C. Mock the external service to record calls without returning data

D. Skip testing the component until the external service is faster

Solution

1. Step 1: Identify the problem with the real service

   The external service is slow, which slows down tests and makes results unpredictable.

2. Step 2: Choose a stub approach for speed and predictability

   Creating a stub that returns fixed data instantly allows fast, reliable tests without depending on the real service.

3. Step 3: Evaluate other options

   Using the real service or skipping tests delays feedback; mocking without return data doesn't help test results.

4. Final Answer:

   Create a stub class that returns fixed, pre-defined data instantly -> Option A

5. Quick Check:

   Stub fixed data = fast, predictable tests [OK]

Hint: Stub fixed data for fast, reliable tests [OK]

Common Mistakes:

• Using real slow service in tests
• Confusing mocks with stubs
• Skipping tests instead of isolating dependencies