The Agile Echo

Best Practices for Test Doubles

A comprehensive guide on test doubles and how they can help you become a better software engineer. Explore the best practices for using test doubles effectively to enhance your software engineering techniques.

Cover Image for Best Practices for Test Doubles
Dan the Dev
Dan the Dev

Best practices for Test Doubles

Welcome to our comprehensive guide on test doubles and how they can help you become a better software engineer. In this article, we will explore the best practices for using test doubles effectively to enhance your software engineering techniques. So, let's dive in and discover how test doubles can take your coding skills to the next level.

Key Takeaways:

  • Become a better software engineer by mastering the use of test doubles
  • Test doubles are software engineering techniques that simulate dependencies in your code
  • Understanding the different types of test doubles, such as mocking, stubs, spies and fakes, is crucial
  • Benefits of test doubles include improved code quality, reduced dependencies, and faster testing
  • Choosing the right test double for a scenario requires careful consideration

Understanding Test Doubles

When it comes to software development, utilizing test doubles can be a game-changer. Test doubles are objects or functions that mimic the behavior of real dependencies in order to facilitate effective testing. They allow you to isolate specific components of your codebase and test them in isolation.

There are different types of test doubles, each serving a specific purpose. Let's explore three common types: mocking, stubs, spies and fakes.


Mocking is a technique where you create objects that simulate the behavior of other objects, typically dependencies. It allows you to define specific behaviors and responses of the mock object during testing. Mocking is particularly useful when you want to verify interactions between objects or when the real dependencies are difficult to instantiate or control during testing.


Stubs, on the other hand, are objects that provide predetermined responses to method calls. Unlike mocks, stubs do not verify interactions. Instead, they return pre-defined values or perform predefined actions when their methods are called. Stubs are helpful when you want to replace expensive or slow dependencies with simple and predictable behavior during testing.


Spies are a lightweight manual implementation of a mock.


Fakes are a lightweight manual implementation of a stub.

Support Me Banner Adv

Benefits of Test Doubles

Incorporating test doubles into your software development workflow offers a multitude of benefits that can significantly improve your code quality, reduce dependencies, and speed up your testing process.

Improved Code Quality

Test doubles play a crucial role in ensuring high code quality by allowing you to test individual components in isolation. By replacing complex dependencies with test doubles, you can focus on testing specific behaviors and edge cases, making it easier to identify and address potential issues. This targeted approach enhances code quality and promotes overall software reliability.

Reduced Dependencies

One of the main advantages of using test doubles is the ability to reduce dependencies on external resources. By simulating the behavior of these dependencies, such as databases, APIs, or third-party services, you can isolate your code from external factors. This independence not only improves the stability of your tests but also makes your code more modular and easier to maintain.

Efficient Testing Process

Test doubles greatly contribute to speeding up the testing process. By replacing slow or time-consuming dependencies with lightweight and controllable test doubles, you can reduce the execution time of your tests. This increased efficiency enables faster iteration cycles, allowing you to quickly identify and fix issues, ultimately accelerating your software development workflow.

"Using test doubles provides numerous benefits, including improved code quality, reduced dependencies, and an efficient testing process. These advantages not only lead to more robust software but also enhance the overall development experience."

Choosing the Right Test Double

When it comes to test-driven development, selecting the right test double is a critical step in ensuring the effectiveness of your tests. By matching the appropriate technique with the specific scenario, you can enhance the accuracy and reliability of your code.

To make an informed choice, consider the following key factors:

  1. Test Coverage: Determine the scope of your test and identify the dependencies that need to be replaced with test doubles. This will guide you in selecting the most suitable technique.
  2. Required Behavior: Understand the behavior your test double needs to emulate. Whether it's mimicking the response of an external API or simulating a specific error condition, selecting a test double that can accurately replicate the desired behavior is crucial.
  3. Isolation: Assess the level of isolation required for your test scenario. Depending on the complexity of your code, you may need to choose between different test double techniques such as mocking, stubbing, or faking to achieve the desired level of isolation.
  4. Maintenance: Evaluate the maintainability of the chosen technique. Consider factors such as ongoing support, ease of updating, and compatibility with future changes to ensure that your test doubles remain effective and efficient over time.

In general, the best approach is to design our software following the Command-Query Separation (CQS) principle, that allow to create only two type of methods in our classes:

  • Commands: methods that mutate the state but does not answer any portion of that state.
  • Queries: methods that answer with the current state (all or a portion of it) but must not modify that state before answering.

When we follow this approach, we can use mocks to replace command methods, and stubs to replace query methods.

By carefully evaluating these factors, you can confidently select the appropriate test double that aligns with your specific scenario. This thoughtful approach will result in more robust tests and enhance the overall quality of your software.

Designing Test Doubles

When it comes to creating effective test doubles, having a solid design is crucial. By following design principles and employing techniques for creating testable code, you can ensure that your test doubles accurately simulate the behavior of dependencies. Let's explore some best practices for designing test doubles that align with the needs of your software development process.

Testable Code: The Foundation for Effective Test Doubles

Before diving into test double design, it's important to establish a foundation of testable code. Testability refers to the ease with which your code can be tested. By writing code that is modular, decoupled, and follows principles such as the Single Responsibility Principle (SRP) and Dependency Inversion Principle (DIP), you can create a solid base for effective test doubles.

"By design, testable code provides the flexibility to replace dependencies with test doubles, allowing for isolated and focused testing of specific code units."

By adhering to solid coding practices such as proper encapsulation, loose coupling, and dependency injection, you can design code that is more easily testable and amenable to using test doubles.

Creating Test Doubles that Accurately Simulate Behavior

When designing test doubles, it's essential to ensure that they accurately simulate the behavior of the dependencies they represent. This requires careful consideration of the interactions and expectations that the code under test has with those dependencies.

One common technique for creating test doubles is to use mocking frameworks, such as Mockito or Sinon, that provide a flexible and intuitive API for simulating behavior. These frameworks allow you to define the expected interactions between your code and its dependencies and verify that the expected behavior is indeed observed.

Another approach to designing test doubles is using handcrafted fakes or spies. These can be specifically designed to mimic the behavior of a particular dependency and provide the desired responses for the code under test. Handcrafted fakes offer more control and can be particularly useful when dealing with complex dependencies that are difficult to mock.

Test Double Type



Creates a replica of the dependency with predefined behaviors and expectations.


Provides predefined responses to specific method calls made by the code under test.


Implements a simplified version of the dependency that closely resembles its behavior but may take shortcuts or simplify certain processes.

Remember, designing effective test doubles requires a deep understanding of your codebase, its dependencies, and the desired behavior. By applying the right design principles and techniques, you can create test doubles that accurately simulate dependencies, improving the reliability and effectiveness of your testing process.

Testing with Test Doubles

When it comes to software development, testing is crucial to ensure the reliability and functionality of your code. Test-driven development (TDD) is an approach that emphasizes writing tests before implementing the actual code. Unit testing and integration testing play integral roles in TDD, where test doubles come into play.

Test doubles are objects or components that replace real dependencies during testing. They allow you to isolate the behavior of individual units of code and simulate the interaction between different components of your software. By incorporating test doubles into your testing process, you can achieve more accurate and reliable test results.

Unit Testing with Test Doubles

Unit testing involves testing individual units or pieces of code in isolation. Test doubles can help you isolate dependencies, such as external services or databases, making unit testing more efficient and focused. Here are some best practices for using test doubles in unit testing:

  • Mocking: Use mock objects to replace a command method, or to simulate the behavior of dependencies and verify the interaction between units of code.
  • Stubs: Use stubs replace a query method, or to provide predetermined responses from dependencies and control the flow of execution during testing.

Remember: behavior doesn't mean implementation!

By employing these techniques, you can create more focused and granular tests that verify the behavior of individual units of code, without the need for complex and time-consuming setups.

Integration Testing with Test Doubles

Integration testing involves testing the interaction between different components or modules of your software. Test doubles can be invaluable in simulating these interactions and ensuring the integrity of your system. Consider the following best practices for using test doubles in integration testing:

  • Mocking: Use mock objects to replace real dependencies and simulate their behavior within the integrated system.
  • Stubs: Use stubs to provide predetermined responses from external dependencies and control the flow of execution during integration testing.
  • Fakes: Use fakes to replace real dependencies that are difficult to handle in an integration environment, streamlining the testing process.

By incorporating test doubles into your integration testing, you can accurately simulate the behavior of external components and identify any potential issues that may arise when they interact with the rest of your system.

Testing Methodology



Unit Testing

Testing individual units of code in isolation

  • Faster execution
  • Isolated testing of specific units
  • Improved code quality and maintainability

Integration Testing

Testing interactions between different components

  • Identifying integration issues
  • Ensuring system integrity
  • Validating system behavior as a whole

In summary, test doubles are powerful tools that can enhance your testing process. By leveraging them effectively in both unit testing and integration testing, you can achieve more accurate and reliable test results, ensure the integrity of your software, and support the principles of test-driven development.

Support Me Banner Adv

Managing Test Doubles

As your codebase evolves, it becomes crucial to implement effective strategies for managing and maintaining your test doubles. By applying proper maintenance techniques and utilizing refactoring methods, you can ensure that your test doubles remain adaptable and enhance the overall maintainability of your application.

Test Double Maintenance

Test double maintenance is an essential aspect of managing your software testing process. Regularly reviewing and updating your test doubles helps to keep them in sync with the changes in your codebase. This ensures that your tests continue to provide accurate results and remain reliable indicators of the system's behavior.

Test double maintenance involves monitoring changes to your system and updating the test doubles accordingly. It requires ongoing collaboration with other team members to ensure that the test doubles accurately simulate the behavior of the dependencies.

By keeping your test doubles up to date, you can effectively identify any issues or discrepancies in the behavior of your application and address them promptly. This proactive approach to test double maintenance helps to prevent potential bugs and ensures the stability and reliability of your software.

Refactoring Techniques

Refactoring test doubles is an essential practice when managing your software testing efforts. Refactoring involves restructuring the code of your test doubles to improve its clarity, maintainability, and overall performance. It allows you to make the necessary changes to accommodate modifications in your application without compromising the integrity of your tests.

Refactoring test doubles involves analyzing the existing code, identifying areas for improvement, and implementing changes to enhance the design and functionality of the test doubles. It focuses on simplifying complex dependencies and making the test doubles more robust and adaptable.

By applying refactoring techniques to your test doubles, you can ensure that they remain relevant and effective throughout the development lifecycle. This not only improves the maintainability of your tests but also enhances your overall software engineering process.

  • Regularly review and update your test doubles to keep them synchronized with changes in your codebase.
  • Collaborate with your team members to ensure the accuracy of your test doubles.
  • Utilize refactoring techniques to improve the clarity and maintainability of your test doubles.
  • Simplify complex dependencies and make your test doubles more adaptable.

By implementing these strategies and techniques, you can effectively manage your test doubles and ensure their longevity and effectiveness in your software development process.

Advanced Test Double Techniques

When it comes to test doubles, there are advanced techniques that can help you handle complex scenarios more effectively. These techniques, namely state verification and behavior verification, allow you to achieve a higher level of confidence in your tests and ensure the accuracy of your software behavior.

State Verification

State verification is a technique that focuses on verifying the state of an object or system after it has undergone certain operations. In the context of test doubles, this involves checking whether the state of the test double matches the expected state based on the actions performed during the test.

This technique is particularly useful when you want to ensure that the test double accurately reflects the behavior of its real counterpart. By verifying the state, you can confirm that the test double is capturing and replicating the appropriate data and internal state changes.

Behavior Verification

Behavior verification, on the other hand, involves verifying that certain behaviors or interactions have occurred between the test double and its dependencies. This technique focuses on checking whether the test double has correctly called the expected methods or received the expected inputs from its dependencies.

Behavior verification is valuable for ensuring that the interactions between objects are behaving as expected. By verifying these interactions, you can ensure that your test doubles are accurately simulating the behavior of the real objects, capturing all necessary dependencies and interactions.

"Advanced test double techniques like state verification and behavior verification provide a deeper level of confidence in your tests. By accurately verifying the state and behavior of your test doubles, you can be confident in the reliability and accuracy of your software behavior." - John Smith, Senior Software Engineer

By incorporating these advanced techniques into your testing process, you can enhance the effectiveness of your tests and gain a greater level of confidence in the quality of your software. Whether you're performing state verification to ensure the accurate replication of internal state or using behavior verification to confirm the proper interactions, these techniques can help you uncover subtle bugs and improve the reliability of your software.

Pitfalls and Challenges with Test Doubles

While test doubles can be incredibly useful in software development, it's important to be aware of their limitations and the common challenges they present. Understanding these pitfalls and knowing how to overcome them is essential for ensuring the effectiveness of your tests.

Test Double Limitations

Test doubles, like any other tool or technique, have their limitations. It's crucial to be conscious of these limitations to avoid potential issues in your testing process. Some common limitations of test doubles include:

1. Limited behavior simulation: Test doubles might not accurately simulate the exact behavior of the dependencies they are replacing. They often provide simplified functionality, which might not cover all possible scenarios or edge cases.

2. Difficulty in handling complex dependencies: Test doubles can struggle when dealing with complex dependencies that involve intricate interactions or state changes.

3. Changes to interfaces: If the interface of a dependency changes, it can break the test doubles that rely on it, requiring updates and adjustments.

4. Divergence from the real implementation: Test doubles may deviate from the actual implementation of a dependency, resulting in false positives or negatives during testing.

Common Challenges

Using test doubles can introduce certain challenges that need to be addressed to ensure the reliability and validity of your tests. Some commonly encountered challenges include:

1. Test data consistency: Ensuring consistent and reliable test data can be challenging when using test doubles. Generating and managing relevant test data across different tests can require careful consideration.

2. Test double maintenance: As your codebase evolves, maintaining and updating test doubles can become complex and time-consuming. Changes in dependencies or interfaces may necessitate significant updates to your test doubles.

3. Balancing isolation and integration: Achieving the right balance between isolating units for testing and ensuring proper integration can be a challenge with test doubles. Overusing or misusing test doubles can lead to overly isolated tests that don't reflect real-world scenarios.

Overcoming these limitations and challenges requires a combination of experience, thorough planning, and continuous improvement. By being aware of these potential issues, you can develop strategies and adopt best practices to mitigate their impact.

Common challenges and limitations are:

  • Difficulty in handling complex dependencies
  • Test double maintenance
  • Changes to interfaces
  • Balancing isolation and integration


Mastering the art of utilizing test doubles effectively is a crucial skill for every software engineer. By incorporating these powerful software engineering techniques into your development workflow, you can become a better software engineer.

Throughout this guide, we've explored the best practices for using test doubles, understanding their different types, and the benefits they bring to your code. We've also discussed how to choose the right test double for your specific scenarios and how to design them to accurately simulate dependencies.

Incorporating test doubles into your testing process, managing and maintaining them, and exploring advanced techniques like state and behavior verification have been key lessons on this journey. By understanding the limitations and challenges associated with test doubles, you can effectively navigate potential pitfalls.

As you apply these best practices and continue to refine your software engineering skills, remember that test doubles are not only a tool but an opportunity for growth. Embrace the power of test doubles and embark on a coding journey that enhances your software engineering skills, elevates your code quality, and drives the success of your projects.

Support Me Banner Adv


What are test doubles?

Test doubles are objects or implementations that mimic the behavior of real dependencies in a software system. They are used in the testing process to isolate the code being tested from its actual dependencies.

What are the different types of test doubles?

There are several types of test doubles, including mocking, stubs, and fakes. Mocking objects are used to verify interactions with dependencies, stubs provide predefined responses to method calls, and fakes are simplified implementations of dependencies.

How can test doubles improve code quality?

Test doubles can improve code quality by allowing developers to test code in isolation, without the need for real dependencies. This helps identify and fix issues early in the development process, leading to more robust and reliable software.

When should I use test doubles?

Test doubles are commonly used in unit testing and integration testing. They are particularly useful when testing code that has dependencies on external systems, databases, or complicated components.

How do I choose the right test double for my scenario?

When choosing a test double, consider the specific requirements of your code and match the technique accordingly. For example, if you need to verify interactions with a dependency, a mocking object would be appropriate.

How can I design effective test doubles?

To design effective test doubles, focus on creating testable code and designing the doubles to accurately simulate the behavior of dependencies. Use design principles that promote modularity and reduce tight coupling between components.

How do test doubles align with test-driven development practices?

Test doubles align with test-driven development (TDD) practices by enabling developers to write tests before writing the actual code. This helps drive the development process and ensures that the code is testable and meets the desired requirements.

What strategies can I use to manage test doubles?

To manage test doubles effectively, consider refactoring techniques that adapt the doubles to changes in your application. Regularly review and update the doubles to maintain their accuracy and improve maintainability.

What are some advanced test double techniques?

Advanced test double techniques include state verification and behavior verification. State verification checks the state changes caused by interactions, while behavior verification verifies that the correct interactions occur.

What are some common challenges with test doubles?

Some common challenges with test doubles include handling complex scenarios, managing the dependencies of the doubles, and ensuring the doubles accurately simulate the behavior of the real dependencies. These challenges can be overcome with careful design and testing practices.

Did you enjoy this post?

Express your appreciations!

Join our Telegram channel and leave a comment!Support Learn Agile Practices

Also, if you liked this post, you will likely enjoy the other free content we offer! Discover it here: