First, the applet would have to know the current session id, so you would need to put the applet in a jsp, and dynamically create an applet parameter that contains the session id.

Then you would need to use a URLConnection to connect to the servlet, and pass the session id to the server, either as a cookie value or using url rewriting.

Once you have done that you can create ObjectInputStream's and ObjectOutputStream's on both the applet and servlet side to pass the required session variables.

Just note, that this would mean the applet is actually going to download the object to the client side, so it might be a bad idea if your application runs over the internet. And this also means your applet would need to be signed. 
Java technology (version 2) is both an object-oriented programming language and a platform developed by Sun Microsystems. Java technology is based on the concept of a single Java virtual machine (JVM) -- a translator between the language and the underlying software and hardware. All implementations of the programming language must emulate the JVM, thus enabling Java programs to run on any system that has a version of the JVM.

The Java programming language is unusual because Java programs are both compiled (translated into an intermediate language called Java bytecodes) and interpreted (bytecodes parsed and run by the JVM). Compilation occurs once, while interpretation happens each time the program is run. Compiled bytecode is a form of optimized machine code for the JVM; the interpreter is an implementation of the JVM. Servlets are java technology's answer to CGI programming. They are programs that run on a Web server and build Web pages. Building Web pages on the fly is useful (and commonly done) for a number of reasons:
The Web page is based on data submitted by the user. For example the results pages from search engines are generated this way, and programs that process orders for e-commerce sites do this as well.
The data changes frequently. For example, a weather-report or news headlines page might build the page dynamically, perhaps returning a previously built page if it is still up to date.
The Web page uses information from corporate databases or other such sources. For example, you would use this for making a Web page at an on-line store that lists current prices and number of items in stock.

you can able to comment this post if you have got any question on the servlets.
A use case captures a contract between the stakholders of a system about it's behavior.The use case describes the system's behavior under various conditions as the system responds to a request from one of the stake holders, called the primary actor.use case are fundamentally a text form, although they can be written using flow charts, sequence charts, petri nets, or programming languages.the use case, as a form of writing , can sitmulate discussion when a team about an upcoming system.

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I love Java and i hate .net, that 's really a garbage i never see before.
"The benevolent see benevolence and the wise see wisdom".