problem 1) You would use the random number generator to find out the arrival of the client. After the client successfully logs in, assign him the initial balance. You might use the same password for every client and lock him out if he fails three times. No other account data is needed for the client and after the client quits using the ATM, you do not have to update/save his account data for next time.
Three transaction options are available: deposit, withdrawal and balance inquiry. And each client could order as many transactions as he wants until he decides to quit. After each transaction, print the receipt showing balances before and after and type and amount of transaction. Error checking on illegal inputs like incorrect transaction option, negative amount and over-withdrawal is required. Design each transaction in a method so the program is better structured.