the easiest example of combining java and flex in my mind will be making a httpservice call in flex app and declare the return format of httpservice call to e4x and in your servlet you return an xml for flex to consume. if you know how to develop java webapp, this can be done pretty fast. More advanced usage will be using amf protocol, Spring has an integration project for BlazeDS framework, which offers message exchange in binary format, this is far performant than exchange data with xml(by means of webservicies)
Do you know why this cup is useful? Because it is empty.