The bank application also manages another sort of account called a checking account. While an ordinary account must maintain a positive balance, a checkingAccount may be overdrawn up to an agreed limit.
We use IDL's notion of interface inheritance to capture the intuition that a checking account is a special form of account:
// in module BankingDemo
interface checkingAccount : account {readonly attribute long limit;
};
The declaration checkingAccount : account specifies that the interface checkingAccount inherits all the operations and attributes declared in the account interface. The body of the definition states that a checkingAccount also supports the additional limit attribute.
The fact that checkingAccount inherits some operations from account does not imply that the methods implementing those operations need to be inherited too. We will exploit this flexibility to provide a specialized debit method for checkingAccounts .