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

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