They have NO common grounds.
A java card needs a real "smart" processor because it contains a Java Virtual Machine, can do crypto and other stuff. The cost to produce one is in the multi dollar range. Some java cards can communicate contactless by "radio".
RFID is the term to describe a very primitive device, all it can do is report a unique identification (ID) and it does this contactless (RF). the cost to produce one is in the cent range. You can use it to track products or containers or other stuff.
Since RFID is also a very popular buzzword these days, the exact meaning of RFID varies.
Of course, you could use a java card to play RFID tag, but that would be
very expensive. Java cards are usually used for bank cards and similar applications.
I will be very appriciated if you could give me clear explainations:
I just confused a few concepts.
Java card technology can be used in contactless smart card(mostly for security). Contactless smart card use rfid technology. Then there must be some applications for java card used in rfid.