Create an interface IAccount and declare the methods getBalance, deposit and withdraw. This interface is implemented by two classes HDFCAccount and State BankAccount, there is difference between implementation of these classes, HDFCAccount uses member variables deposits and withdrawals for maintaining the balance, where as State BankAccount uses only balance to maintain the balance. In the main method create objects of HDFCAccount and State BankAccount, but assign them to the reference of the interface IAccount. The methods transactOnAccount and printBalance only know about the interface IAccount and does not getting impacted on which class's object is passed. Use java