[JUnit] is the de facto standard regression testing framework for Java. It allows to write and run unit tests for Java code. JUnit separates test code from production code and integrates well into build process. Unit tests ensure correctness of the code, thus making the code more stable. Unit tests also constitute a form of design documentation, determine when and how much of the code is done, and give confidence to the developers.
JUnit is Open Source Software and is available free of charge to Java developers.