We might model such a scenario using three types of object: one for Customers, one for BankAccounts and another for Transactions. In terms of data required, for Customers assume we need a name, an address and a savings account. An array of Customers should be used. For the savings account assume we require an account number, a balance (amount) and an interest rate. For transactions  assume we need an account number the transaction applies to, an amount being transacted and a transaction type (such as Deposit, Withdrawal or Interest ). Details of the transactions are held in a file called "trans.dat". Below is a suggested UML type diagram representing the object types (classes):