Unit testing is a software development process in which the smallest testable parts of an application, called units, are individually and independently scrutinized for proper operation. Unit testing can be done manually but is often automated.
Unit testing is a component of test-driven development (TDD), a pragmatic methodology that takes a meticulous approach to building a product by means of continual testing and revision. Test-driven development requires that developers first write failing unit tests. Then they write code and refactor the application until the test passes. TDD typically results in an explicit and predictable code base.
Unit testing involves only those characteristics that are vital to the performance of the unit under test. This encourages developers to modify the source code without immediate concerns about how such changes might affect the functioning of other units or the program as a whole. Once all of the units in a program have been found to be working in the most efficient and error-free manner possible, larger components of the program can be evaluated by means of integration testing.
Unit testing does have steep learning curve. The development team needs to learn what unit testing is, how to unit test, what to unit test and how to use automated software tools to facilitate the process on an on-going basis. The great benefit to unit testing is that the earlier a problem is identified, the fewer compound errors occur. A compound error is one that doesn't seem to break anything at first, but eventually conflicts with something down the line and results in a problem.
This Google TechTalk provides an overview of unit testing.
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