JavaCard OS

I am working on applet development for online payment. Now I am experiencing a problem about how to store the offline data into java card.
How can I do the offline transactions? Hope somebody provide me more points. thx

You should store the offline balance in the persistent memory. Moreover, updates to this persistent memory must be atomic.
That is if a transaction is going on and there is a card tear, then the transaction should be invalid.
For this, Java Card has the beginTransaction and abortTransaction methods in JCSystem.