Java Programming

I have a question and i dont undersatnd what should i write the code for this program.If any link related to this topic then give me.Please help

Q)
write compiling code for Cash withdrawal function from ATM which based on user specified amount,dispenses bank notes :
Ensure that code takes care of following things
1) minimun number of bank notes are used while despensing the amount
2)availability of various denominations in the atm machine is maintained.
3)code should take care of any bank denomination as long as it is multiple of 10.
4)code should support parallel cash withdrawal means more tha one person can withdraw money at the same time.

Thanks
Sumit