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.
Once you have the library installed you can use this application as an example (extract the zip-file and import it into Ecplise via File > Import > Existing projects into Workspace).
The example consists of a class
SerialComm which handles the communication and a class
SerialGUI which is a simple userinterface with a label which shows a welcome message if a know RFID is scanned: