Junit

Chances are good that you're already using JUnit. If so, you can skip ahead to the next section. If you're not using JUnit, you need to be. JUnit is an automated testing framework that lets you build simple tests. You can then execute each test as part of the build process, so you know immediately when something breaks. At first, most developers resist unit testing because it seems like lots of extra work for very little benefit. They dig in their heels. Automated unit testing is foundational:
JUnit testing lets you run every test, with every build.
JUnit testing gives you the courage to try new things.
JUnit lets you save and use debugging code that you're going to write anyway.
JUnit forces you to build better code.

The above is from this book: Better, Faster, Lighter Java
Reference: http://junit.org