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In computer programming, unit testing is a procedure used to validate that individual units of source code are working properly. A unit is the smallest testable part of an application. In procedural programming a unit may be an individual program, function, procedure etc, while in object-oriented programming, the smallest unit is always a Class; which may be a base/super class, abstract class or derived/child class. Wikipedia
The testing process will take us through a journey of exploration of our software in order to determine its quality, if it met the gathered requirements, work as expected, and provide the required functionality. However, all the testing that we do is mostly controlled and under certain conditions. It is extremely difficult to determine all factors and all software conditions. Therefore attaining a testing nirvana is unrealistic.