What is the difference between a test driver and a test stub?

In software testing, both test drivers and test stubs are used to facilitate the testing process, particularly in the context of integration testing. They are components of the incremental testing approach, where individual units or modules are tested together to verify their interactions and integration. Let's understand the difference between test drivers and test stubs:

Test Driver:

A test driver is a component used during bottom-up integration testing. It is responsible for simulating the behavior of a higher-level module that calls or interacts with the module being tested (the one just below it in the hierarchy). The purpose of the test driver is to provide a controlled environment to test the integration of the lower-level module with the higher-level module.

In essence, the test driver acts as a "fake" higher-level module and provides the necessary input and environment to execute the code in the lower-level module. It helps in evaluating the correctness of the lower-level module's behavior when integrated into the larger system.

Test Stub:

On the other hand, a test stub is used during top-down integration testing. It is employed to simulate the behavior of a lower-level module that is called by the module being tested (the one just above it in the hierarchy). The purpose of the test stub is to provide a controlled response to the calls from the higher-level module.

In simple terms, the test stub acts as a "dummy" lower-level module, and it is implemented to return specific predefined values or responses to the higher-level module when called. This allows the higher-level module to be tested independently of the actual lower-level module, which might not be ready or fully implemented yet.

In summary, the key difference between a test driver and a test stub lies in their use cases and roles during integration testing:

Test Driver: Used for bottom-up integration testing, simulates higher-level module behavior, and provides input and environment for testing lower-level modules.

Test Stub: Used for top-down integration testing, simulates lower-level module behavior, and provides predefined responses to calls from higher-level modules.

Both test drivers and test stubs aid in incremental integration testing, where individual units are combined and tested in stages to ensure that they work together as expected within the larger system.