Mockito BDDMockito thenAnswer Method

The thenAnswer method in the BDDMockito class is part of the Behavior-Driven Development (BDD) style of writing tests using Mockito. It allows you to specify custom behavior for a method call on a mock object. This method is particularly useful for making tests more readable and aligning with the BDD style, which focuses on describing the behavior of the application in a clear and human-readable format.

Table of Contents

  1. Introduction
  2. thenAnswer Method Syntax
  3. Examples
    • Basic Usage
    • Using thenAnswer with Complex Logic
    • Using thenAnswer to Return Different Values
  4. Real-World Use Case
  5. Conclusion

Introduction

Behavior-Driven Development (BDD) is a software development approach that emphasizes collaboration between developers, QA, and non-technical or business participants in a software project. Mockito’s BDDMockito class provides methods that support the BDD style of writing tests. The thenAnswer method is used to specify custom behavior for method calls on mock objects, making tests more readable and aligning with the BDD approach.

thenAnswer Method Syntax

Specifying a Custom Answer

import org.mockito.BDDMockito;
import org.mockito.stubbing.Answer;

static <T> BDDMockito.BDDMyOngoingStubbing<T> willAnswer(Answer<?> answer)

Specifies custom behavior for a method call on a mock object.

Parameters:

  • answer: The custom behavior to be executed when the method is called.

Returns:

  • A BDDMyOngoingStubbing object that allows further stubbing.

Examples

Basic Usage

Use thenAnswer to specify a basic custom behavior for a method call on a mock object.

import static org.mockito.BDDMockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class BasicThenAnswerTest {
    @Test
    void testThenAnswer() {
        UserService mockUserService = mock(UserService.class);

        // Set up the custom behavior
        willAnswer(new Answer<String>() {
            @Override
            public String answer(InvocationOnMock invocation) {
                String userId = invocation.getArgument(0);
                return "Mock user details for " + userId;
            }
        }).given(mockUserService).getUserDetails("user123");

        // Call the method
        String result = mockUserService.getUserDetails("user123");

        // Verify the result
        assertEquals("Mock user details for user123", result);
    }
}

class UserService {
    public String getUserDetails(String userId) {
        return "Real user details for " + userId;
    }
}

Using thenAnswer with Complex Logic

Use thenAnswer to specify custom behavior with complex logic for a method call on a mock object.

import static org.mockito.BDDMockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class ComplexThenAnswerTest {
    @Test
    void testThenAnswerWithComplexLogic() {
        MathService mockMathService = mock(MathService.class);

        // Set up the custom behavior with complex logic
        willAnswer(new Answer<Integer>() {
            @Override
            public Integer answer(InvocationOnMock invocation) {
                int a = invocation.getArgument(0);
                int b = invocation.getArgument(1);
                return a * b;
            }
        }).given(mockMathService).multiply(anyInt(), anyInt());

        // Call the method
        int result = mockMathService.multiply(5, 3);

        // Verify the result
        assertEquals(15, result);
    }
}

interface MathService {
    int multiply(int a, int b);
}

Using thenAnswer to Return Different Values

Use thenAnswer to specify custom behavior that returns different values based on the input arguments.

import static org.mockito.BDDMockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class ThenAnswerDifferentValuesTest {
    @Test
    void testThenAnswerDifferentValues() {
        UserService mockUserService = mock(UserService.class);

        // Set up the custom behavior to return different values
        willAnswer(new Answer<String>() {
            @Override
            public String answer(InvocationOnMock invocation) {
                String userId = invocation.getArgument(0);
                if ("user123".equals(userId)) {
                    return "Mock user details for user123";
                } else if ("user456".equals(userId)) {
                    return "Mock user details for user456";
                }
                return "Unknown user";
            }
        }).given(mockUserService).getUserDetails(anyString());

        // Call the method with different arguments
        String result1 = mockUserService.getUserDetails("user123");
        String result2 = mockUserService.getUserDetails("user456");
        String result3 = mockUserService.getUserDetails("user789");

        // Verify the results
        assertEquals("Mock user details for user123", result1);
        assertEquals("Mock user details for user456", result2);
        assertEquals("Unknown user", result3);
    }
}

class UserService {
    public String getUserDetails(String userId) {
        return "Real user details for " + userId;
    }
}

Real-World Use Case

Simplifying Tests for Services with Custom Behaviors

In a real-world scenario, you might need to test services with methods that have custom behaviors. Using thenAnswer can simplify these tests by allowing you to specify the custom behaviors in a readable and intuitive way.

import static org.mockito.BDDMockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

interface NotificationService {
    void sendNotification(String recipient, String message);
}

class UserService {
    private final NotificationService notificationService;

    public UserService(NotificationService notificationService) {
        this.notificationService = notificationService;
    }

    public void notifyUser(String userId, String message) {
        notificationService.sendNotification(userId, message);
    }
}

public class UserServiceTest {
    @Test
    void testNotifyUserWithCustomBehavior() {
        NotificationService mockNotificationService = mock(NotificationService.class);
        UserService userService = new UserService(mockNotificationService);

        // Set up the custom behavior
        willAnswer(new Answer<Void>() {
            @Override
            public Void answer(InvocationOnMock invocation) {
                String recipient = invocation.getArgument(0);
                String message = invocation.getArgument(1);
                System.out.println("Sending notification to " + recipient + ": " + message);
                return null;
            }
        }).given(mockNotificationService).sendNotification(anyString(), anyString());

        // Call the method
        userService.notifyUser("user123", "Your account has been updated.");

        // Verify the interaction
        then(mockNotificationService).should().sendNotification("user123", "Your account has been updated.");
    }
}

In this example, the UserServiceTest class uses Mockito’s BDDMockito.given method along with thenAnswer to set up custom behavior for the sendNotification method. This simplifies the test by allowing you to specify the custom behavior and verify the interaction in a readable and intuitive way.

Conclusion

The BDDMockito.thenAnswer method in Mockito is used for specifying custom behaviors for method calls on mock objects in a BDD style. By using thenAnswer, you can make your tests more readable and align them with the BDD approach, focusing on the behavior of the application rather than the implementation details. This helps ensure that your tests are clear, comprehensive, and easy to understand.

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