This example presumes you already have some basic knowledge about Arduino and programming Java applications with Eclipse.
If you have an RFID reader module attached to your computer (via a USB serial connection) you can read RFID data from a Java application. For instance, you can use the unique id (Card UID) to identify something or someone.
To use serial communication in Java you need a library like jSerialComm. If you are starting a new project, download the .jar file, copy it to the root folder of your Eclipse project and import it (right click it and select Build Path > Add to Build Path).
You can use this application as an example (extract the zip-file and import it into Ecplise via File > Import > Existing projects into Workspace). It already contains the jSerialComm library.
The example has a class
SerialGUI which is a simple userinterface with a label which shows a welcome message if a know RFID is scanned: