Test-Driven Development is related to the test-first programming concepts of Extreme Programming, begun in the late 20th century, but more recently is creating more general interest in its own right.
Along with other techniques, the concept can also be applied to the improvement and removal of software defects from legacy code that was not developed in this way .
The new test should also fail for the expected reason. This step tests the test itself, in the negative: it rules out the possibility that the new test will always pass, and therefore be worthless.
It is important that the code written is only designed to pass the test; no further (and therefore untested) functionality should be predicted and 'allowed for' at any stage.
To achieve some advanced design concept (such as a Design Pattern), tests are written that will generate that design. The code may remain simpler than the target pattern, but still pass all required tests. This can be unsettling at first but it allows the developer to focus only on what is important.
Test-driven development requires the programmer to first fail the test cases. The idea is to ensure that the test really works and can catch an error. Once this is shown, the normal cycle will commence. This has been coined the "Test-Driven Development Mantra", known as red/green/refactor where red means fail and green is pass.
Test-driven development constantly repeats the steps of adding test cases that fail, passing them, and refactoring. Receiving the expected test results at each stage reinforces the programmer's mental model of the code, boosts confidence and increases productivity.
Advanced practices of test-driven development can lead to Acceptance Test-driven development [ATDD] where the criteria specified by the customer are automated into acceptance tests, which then drive the traditional unit test-driven development [UTDD] process. This process ensures the customer has an automated mechanism to decide whether the software meets their requirements. With ATDD, the development team now has a specific target to satisfy, the acceptance tests - which keeps them continuously focused on what the customer really wants from that user story.
Programmers using pure TDD on new ("greenfield") projects report they only rarely feel the need to invoke a debugger. Used in conjunction with a Version control system, when tests fail unexpectedly, reverting the code to the last version that passed all tests may often be more productive than debugging.
Test-driven development can help to build software better and faster. It offers more than just simple validation of correctness, but can also drive the design of a program. By focusing on the test cases first, one must imagine how the functionality will be used by clients (in this case, the test cases). Therefore, the programmer is only concerned with the interface and not the implementation. This benefit is complementary to Design by Contract as it approaches code through test cases rather than through mathematical assertions or preconceptions.
The power test-driven development offers is the ability to take small steps when required. It allows a programmer to focus on the task at hand as the first goal is to make the test pass. Exceptional cases and error handling are not considered initially. Tests to create these extraneous circumstances are implemented separately. Another advantage is that test-driven development, when used properly, ensures that all written code is covered by a test. This can give the programmer, and subsequent users, a greater level of trust in the code.
While it is true that more code is required with TDD than without TDD because of the unit test code, total code implementation time is typically shorter. Large numbers of tests help to limit the number of defects in the code. The early and frequent nature of the tests helps to catch defects early in the development cycle, preventing them from becoming endemic and expensive problems. Eliminating defects early in the process usually avoids lengthy and tedious debugging later in the project.
TDD can lead to more modularized, flexible, and extensible code. This effect often comes about because the methodology requires that the developers think of the software in terms of small units that can be written and tested independently and integrated together later. This leads to smaller, more focused classes, looser coupling, and cleaner interfaces. The use of the Mock Object design pattern also contributes to the overall modularization of the code because this pattern requires that the code be written so that modules can be switched easily between mock versions for unit testing or "real" version for deployment.
Because no more code is written than necessary to pass a failing test case, automated tests tend to cover every code path. For example, in order for a TDD developer to add an else branch off an existing if branch, the developer would first have to have written a failing test case that motivates the branch. As a result, the automated tests developed through strict application of TDD tend to be very robust, and can detect any significant functional mutation to the code base.
Test-suite code clearly has to be able to access the code it is testing. In almost every case imaginable, this access occurs through the published interface of function, procedure, or method calls. The use of "mock objects" ensures information hiding remains intact, guaranteeing a total separation of concerns.
Unit test code for TDD is almost never written within the same project or module as the code being tested. By placing tests in a separate module or library, the production code remains pristine. Placing the TDD code inside the same module would fundamentally alter the production code. Use of conditional compilation directives can introduce subtle bugs.
Some may object that using strict black box testing does not provide access to
private data and methods. This is intentional; as the software evolves, you may find the implementation of a class changes fundamentally. Remember a critical step of test-driven development is to refactor. Refactoring may introduce changes which adds or removes private members, or alters an existing member's type. These changes ought not break existing tests. Unit tests that exploit glass box testing are highly coupled to the production software; changing the implementation of a class or module may mean you must also update or discard existing tests, things which should never have to occur. For this reason, glass box testing must be kept to the minimum possible. White box testing should never be used in test-driven development.
In all cases, thought must be given to the question of deployment. The best approach is to develop your software so that you have three major components. The first major component is the unit test runner application framework itself. The second is the main entry module for the production logic. Both of these modules would link (preferably dynamically) to one or more libraries, each implementing some or all of the business logic under development. This guarantees total modularity and is thoroughly deployable.
When code under development relies on a database or a web service or any other external process or service, enforcing a unit-testable separation is an opportunity and a driving force to design more modular, more testable and more re-usable code. Two steps are necessary:
A corollary of this approach is that the actual database or other external-access code is never tested by the TDD process itself. To avoid this, other tests are needed that instantiate the test-driven code with the 'real' implementations of the interfaces discussed above. Many developers find it useful to keep these tests quite separate from the TDD unit tests, and refer to them as integration tests. There will be fewer of them, and they need be run less often than the unit tests. They can nonetheless be implemented using the same testing framework, for example xUnit.
Integration tests that alter any persistent store or database should always be careful to leave them in a state ready for re-use, even if any test fails. This can be achieved using some combination of the following techniques where relevant and available to the developer:
TearDownmethod integrated into many test frameworks
try...catch...finallyexception handling structures where available
Frameworks such as jMock and Rhino Mocks exist to make the process of creating and using complex mock objects easier.
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