Rupiz: A basic design

What do we need for the back end of a personal finance application?

• Some way to represent Accounts. An Account has a description, current balance, and some methods to modify the current balance.


• We want to encapsulate modifications to accounts in Transactions, which would have a description, source account, destination account, and amount. In addition, we will want a version of Transaction that holds a number of subtransactions.


• We want users of our Account class to not care how the account is implemented (in particular how the current balance is represented). To make this possible, we will define an Amount interface to hide these details. To start, we will use an implementation that uses BigDecimal to hold the amount info. If we want to change this later, we can do so easily and without a lot of search-and-replacing.

So for our model, we will have an Account class that holds a BigDecimal for the current balance. This Account will have methods deposit(Amount) and withdraw(Amount) that both return an implementation of Amount representing the updated account balance. To track our account balance changes, we will have a Transaction interface with implementations BasicTransaction and CompoundTransaction, where CompoundTransaction will allow us to group a number of transactions together. We will maintain a List<Transaction> for each account we're dealing with.

For our GUI, we will need a way to show our list of Transactions for a given account. A JTable will do nicely until we add support for compound transactions. Our JTable will need columns for Date, Description, Transaction Type, Source/Destination, Amount, and whether the transaction has been reconciled.