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In http://en.wikipedia.org/wiki/Computer_programming - computer programming , unit testing is a procedure used to validate that individual http://en.wikipedia.org/wiki/Module_%28programming%29 - modules or units of http://en.wikipedia.org/wiki/Source_code - source code are working properly.
More technically one should consider that a unit is the smallest
testable part of an application. In a Procedural Design a unit may be
an individual program, function, procedure, web page, menu etc. But in
Object Oriented Design, the smallest unit is always a Class; which may
be a base/super class, abstract class or derived/child class.
A unit test is a test for a specific unit. Ideally, each http://en.wikipedia.org/wiki/Test_case - test case is independent from the others; http://en.wikipedia.org/wiki/Mock_object - mock objects and http://en.wikipedia.org/wiki/Test_harness - test harnesses can be used to assist testing a module in isolation. Unit testing is typically done by the http://en.wikipedia.org/wiki/Software_developer - developers and not by http://en.wikipedia.org/wiki/End-user - end-users .
The goal of unit testing is to isolate each part of the program and
show that the individual parts are correct. Unit testing provides a
strict, written contract that the piece of code must satisfy. As a
result, it affords several benefits.
Unit testing allows the programmer to http://en.wikipedia.org/wiki/Refactoring - refactor code at a later date, and make sure the module still works correctly (i.e. http://en.wikipedia.org/wiki/Regression_testing - regression testing ). The procedure is to write test cases for all http://en.wikipedia.org/wiki/Subroutine - functions and http://en.wikipedia.org/wiki/Method_%28computer_science%29 - methods
so that whenever a change causes a regression, it can be quickly
identified and fixed. This provides the benefit of encouraging
programmers to make changes to the code since it is easy for the
programmer to check if the piece is still working properly. Good unit
test design produces test cases that http://en.wikipedia.org/wiki/Code_coverage - cover all paths through the unit with attention paid to loop conditions.
In continuous unit testing environments, through the inherent
practice of sustained maintenance, unit tests will continue to
accurately reflect the intended use of the executable and code in the
face of any change. Depending upon established development practices
and unit test coverage up-to-the-second accuracy can be maintained.
Unit testing helps to eliminate uncertainty in the units themselves and can be used in a http://en.wikipedia.org/wiki/Top-down_and_bottom-up_design - bottom-up testing style approach. By testing the parts of a program first and then testing the sum of its parts, http://en.wikipedia.org/wiki/Integration_testing - integration testing becomes much easier.
A heavily debated matter exists in assessing the need to perform
manual integration testing. While an elaborate hierarchy of unit tests
may seem to have achieved integration testing, this presents a false
sense of confidence since integration testing evaluates many other
objectives that can only be proven through the human factor. Some argue
that given a sufficient variety of test http://en.wikipedia.org/w/index.php?title=Automation_systems&action=edit - automation systems ,
integration testing by a human test group is unnecessary.
Realistically, the actual need will ultimately depend upon the
characteristics of the product being developed and its intended uses.
Additionally, the human or manual testing will greatly depend on the
availability of resources in the organization.
Unit testing provides a sort of "living document". Clients
and other developers looking to learn how to use the module can look at
the unit tests to determine how to use the module to fit their needs
and gain a basic understanding of the http://en.wikipedia.org/wiki/Application_programming_interface - API .
Unit http://en.wikipedia.org/wiki/Test_case - test cases
embody characteristics that are critical to the success of the unit.
These characteristics can indicate appropriate/inappropriate use of a
unit as well as negative behaviors that are to be trapped by the unit.
A unit test case, in and of itself, documents these critical
characteristics, although many software development environments do not
rely solely upon code to document the product in development.
On the other hand, ordinary narrative documentation is more
susceptible to drifting from the implementation of the program and will
thus become outdated (e.g. design changes, feature creep, relaxed
practices to keep documents up to date).
Because some classes may have http://en.wikipedia.org/wiki/Reference_%28computer_science%29 - references
to other classes, testing a class can frequently spill over into
testing another class. A common example of this is classes that depend
on a http://en.wikipedia.org/wiki/Database - database :
in order to test the class, the tester often writes code that interacts
with the database. This is a mistake, because a unit test should never
go outside of its own class boundary. As a result, the software
developer abstracts an interface around the database connection, and
then implements that interface with their own http://en.wikipedia.org/wiki/Mock_object - mock object .
By abstracting this necessary attachment from the code (temporarily
reducing the net effective coupling), the independent unit can be more
thoroughly tested than may have been previously achieved. This results
in a higher quality unit that is also more maintainable. In this
manner, the benefits themselves begin returning dividends back to the
programmer creating a seemingly perpetual upward cycle in quality.
Unit testing will not catch every error in the program. By
definition, it only tests the functionality of the units themselves.
Therefore, it will not catch integration errors, http://en.wikipedia.org/wiki/Performance_testing - performance
problems or any other system-wide issues. In addition, it may not be
easy to anticipate all special cases of input the program unit under
study may receive in reality. Unit testing is only effective if it is
used in conjunction with other http://en.wikipedia.org/wiki/Software_testing#Software_testing_activities - software testing activities .
It is unrealistic to test all possible input combinations for any
non-trivial piece of software. Like all forms of software testing, unit
tests can only show the presence of errors; it cannot show the absence
of errors.
To obtain the intended benefits from unit-testing, a rigorous sense
of discipline is needed throughout the software development process. It
is essential to keep careful records, not only of the tests that have
been performed, but also of all changes that have been made to the
source-code of this or any other unit in the software. Use of a
so-called "version control system" is essential: if a later version of
the unit fails a particular test that it had previously passed, the
version-control software can provide list of the source-code changes
(if any) that have been applied to the unit since that time.
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