The Java UI code generated by RapidClipse is based on the Vaadin framework. Vaadin consists of a client side and a server side.
Source: Vaadin documentation.
The server side of Vaadin allows you to develop the entire interface in Java. In this regard, Vaadin is very similar to Java Swing. The UI code is automatically generated by the RapidClipse GUI Builder so that you don't have to write it by hand. At runtime, the server-side Vaadin application runs in a Servlet container on the server, delivers the interface to the client, handles the User Session Management and the requests of individual clients, and automatically updates the interface in the event of changes, so that you do not have to perform any of these tasks manually.
The design and appearance of the interface can be defined using CSS or SCSS. Several high-quality themes are available with Vaadin by default.
Google Web-Tookit (GWT)