A word of warning, there is plenty of google brain cancer in GWT, one of my personal favorites that elicited "wtf..."s frequently in a previous occupation:

http://www.gwtproject.org/javadoc/latest/com/google/gwt/json...

To this day I still can't fathom why someone thought it'd be acceptable to violate conventions by providing is<Type>() methods that do not return boolean results.