Mockito BDDMockito willAnswer()

1. Overview

The BDDMockito willAnswer() function plays an essential role in shaping Behavior-Driven Development (BDD) style unit tests in Mockito. Unlike willReturn(), which provides a fixed return value, willAnswer() grants more flexibility by allowing the provision of dynamic responses using the Answer interface. This tutorial will walk through the usage of willAnswer() in the context of BDD testing with Mockito.

2. Development Steps

1. Set up a Maven project.

2. Add the required Mockito and JUnit 5 dependencies.

3. Design a CalculatorService class, our object under test.

4. Develop a BDD-style test for CalculatorService, utilizing BDDMockito's willAnswer().

5. Run the test and review the results.

6. Discuss the importance and benefits of the willAnswer() method within BDD testing.

3. Dependencies (Mockito and JUnit 5)

<!-- JUnit 5 -->
<!-- Mockito Core -->

4. Code Program

// A simple CalculatorService class for demonstration
class CalculatorService {
    double multiply(double a, double b) {
        // In a real scenario, this might be a more complex operation
        return a * b;
// BDD-style test class for CalculatorService using BDDMockito's willAnswer()
import org.junit.jupiter.api.Test;
import org.mockito.invocation.InvocationOnMock;
import static org.mockito.BDDMockito.*;
public class CalculatorServiceBDDTest {
    public void testMultiply() {
        // Given: Setting up the scenario
        CalculatorService mockCalculator = mock(CalculatorService.class);
        // Using willAnswer() to provide a dynamic response
        willAnswer(invocation -> {
            double arg1 = invocation.getArgument(0);
            double arg2 = invocation.getArgument(1);
            return arg1 * 2 + arg2 * 2;
        }).given(mockCalculator).multiply(anyDouble(), anyDouble());
        // When: Executing the action
        double result = mockCalculator.multiply(2, 3);
        // Then: Asserting the expected outcome
        System.out.println("Multiplication Result: " + result);


Multiplication Result: 10.0

Code Explanation:

1. We started with the CalculatorService class, containing the multiply() method.

2. The test class CalculatorServiceBDDTest uses a BDD approach with BDDMockito.

3. Within the test, willAnswer() is leveraged to dynamically compute the return value for the mocked multiply() method. Instead of simply returning a constant value, the method processes its input arguments, showcasing the flexibility of willAnswer().

4. In the "When" section, we call the multiply() method on our mock, expecting our custom logic to execute.

5. The result is then displayed in the output section, confirming our mock's behavior.

5. Conclusion

BDDMockito's willAnswer() function endows developers with the power to craft dynamic responses in BDD-style unit tests. While simpler methods like willReturn() have their place, willAnswer() stands out when you need to accommodate more intricate, argument-dependent responses. 

Embracing willAnswer() can lead to more flexible and adaptable tests, ensuring that our mock behaviors can adapt to a variety of test scenarios.

Related Mockito BDDMockito Class Methods (Behavior-Driven Development Style)

Mockito BDDMockito
Mockito BDDMockito given()
Mockito BDDMockito willThrow()
Mockito BDDMockito willAnswer()
Mockito BDDMockito willReturn()
Mockito BDDMockito willDoNothing()
Mockito BDDMockito willCallRealMethod()
Mockito BDDMockito then()
Mockito BDDMockito.any()
Mockito BDDMockito.times()