Module Package Restrictions

While GWT modules may appear anywhere in your classpath and use Java’s package naming convention, there are restrictions as to where source code can be placed inside of a GWT module. If you want to add some new classes, you can’t just add them anywhere. Java classes must be placed in the client sub-package, in a package that is listed in the source path, or in the server sub-package. The placement of code into these packages is not arbitrary, it is dependent on what you intend to do with your classes.
Client-side code must always be included in either the client sub-package or in a package listed in the source path
Client-side code is code that you expect to be translated into JavaScript and executed in a web-browser. For example, if you create a GWT UI for your application, all the GWT UI classes and the classes they interact with are considered to be client-side code since they must be translated to JavaScript before you can access them through a web-browser. Any code that needs to be translated to JavaScript and executed on the client side must always be included in either the client sub-package or in a package listed in the source path. You also need to ensure that this client-side source code is compatible with Java 1.4.2 or older and only uses classes provided by the GWT JRE Emulation library.

Server-side code must always be included in the server sub-package
Server-side code is code that you expect to be executed on a server computer. This code is executed as is. For example, server side implementations of GWT RPC services, or any other classes that interacts with the GWT RPC services are considered to be server-side code since they execute on a server. Any code that executes on a server as part of a GWT application must always be included in the server sub-package. None of the client-side language restrictions apply to server-side code.
NOTE: If you have classes that you wish to use in both client and server-side code, these classes need to be translated to JavaScript for use in the client. Therefore classes that you wish to use in both client and server-side code should be included in either the client sub-package or in a package listed in the source path. All client-side code language restrictions apply to these classes.