To make the Java files available to the GWT compiler you need to
Create a gwt.xml file in the Java project / jar file which you want to use - This will instruct the GWT compiler to use the listed classes.
Use the included library via the inherit definition
If you are using a jar file you also need to include the source files in the jar
From:
http://www.vogella.de/articles/GWTModules/article.html
If you don't have access to the
Java project/jar file, or don't want to explicitly link it to GWT, you can include the gwt.xml file in the same package within your GWT project rather than within the other project.