Groovy has full access to the Java API plus some other really cool extra API stuff of its own.
If you've used Swing before, you may want to check out Groovy's
Swing Builder which makes the code for building UI components more declarative, which creates less code clutter and is easier to read.
Speaking of builders, check out other
Builders, especially scroll down to see how easy it is to write HTML and XML using Groovy!