Test Early :

One of the nice aspects of Google’s Web Tool kit is its built-in developer testing functionality à la JUnit. Not only does GWT give you the ability to develop Ajax components within Java (as opposed to JavaScript) but by standardizing the development platform on Java, they’ve facilitated the use of testing Ajax related code with what’s arguably the standard testing platform for Java developers. One key thing to keep in mind, however, is that Google’s GWTTestCase isn’t meant to test UI related code– it’s meant to facilitate testing asynchronous aspects that can be triggered by UI interactions.

Testing GWT with JUnit