Techniques for Creating Test Doubles: Mocks, Stubs, and Fakes

In the world of Test Driven Development (TDD), test doubles play a crucial role. They help isolate the code under test and ensure that each component functions correctly in isolation. Test doubles can simulate the behavior of dependencies, allowing developers to write focused, reliable tests. In this article, we will explore three commonly used techniques for creating test doubles: mocks, stubs, and fakes.


Mocks are objects that simulate the behavior of real objects in controlled ways. They record the interactions between the code under test and the dependencies, tracking method invocations, parameter values, and return values. By defining expectations on these interactions, mocks validate whether the code under test interacts correctly with its dependencies.

To create a mock, developers can use testing frameworks or libraries that provide built-in mock functionalities. These frameworks allow developers to define the expected behavior of each method call on the mock object, including specifying return values or throwing exceptions. By comparing the expected interactions with the actual interactions, developers can ensure that the code under test behaves as intended.

Mocks are useful in scenarios where the code under test has complex interactions with its dependencies. They allow developers to verify that specific methods are called with expected parameters, or to ensure that certain methods are not called at all. Mocks can also be used to simulate error conditions or exceptional behaviors from dependencies, helping developers test the resilience of their code.


Unlike mocks, stubs focus on providing canned responses instead of tracking interactions. Stubs simulate the behavior of dependencies but do not assert or validate how they are used. They are simpler to create and use, making them a popular choice for test doubles.

Stubs are pre-programmed objects that return pre-determined values when specific methods are called. They allow developers to control the behavior of dependencies during tests, ensuring that the code under test receives the expected data or triggers specific conditions. Stubs provide a predictable environment, making tests more reliable and deterministic.

To create a stub, developers can implement classes that inherit or implement the same interfaces as the dependencies they are substituting. By overriding the methods of the interface, developers can define the desired behavior. Alternatively, testing frameworks and libraries often provide utilities for creating stubs.

Stubs are suitable when developers want to test specific scenarios and need control over the data provided by dependencies. They can be especially helpful in testing error handling, edge cases, or states that are difficult to reproduce with real dependencies.


Fakes differ from mocks and stubs as they are functional implementations of dependencies. While mocks and stubs simulate behaviors, fakes are simplified implementations that provide the same functional capabilities as the real dependencies. Fakes are typically simpler and more lightweight, making them easier to maintain and understand.

Developers create fakes by implementing simplified versions of the dependencies, focusing on delivering the core functionality required for testing. Unlike stubs, fakes are not limited to canned responses but can have their own behavior and logic. Fakes can be developed specifically for testing purposes or can be existing implementations that are modified for testing.

Fakes are beneficial when simulating the behavior of complex dependencies that are difficult to control or set up properly. They allow developers to test their code in an isolated environment, reducing dependencies on external systems and improving test reliability. However, it is crucial to ensure that the fakes accurately replicate the behavior of the real dependencies to guarantee reliable test results.


Test doubles are essential for effective test-driven development. By utilizing techniques like mocks, stubs, and fakes, developers can isolate their code and verify its behavior in a controlled environment. Each technique offers unique benefits, and the choice depends heavily on the specific requirements of the code under test. Incorporating these techniques into TDD practices can significantly enhance the quality and reliability of software development.

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