Account.setAnnualInterestRate is different from account.setAnnualInterestRate.
I recommend you to study "static"
All accounts have the same interest rate.
--> If every account has its own annualInterestRate, it's waste of memory. (declare annualInterestRate as default)
--> It's better to store annualInterestRate only once regardless of the number of accounts. (declare annualInterestRate as static)
That's why the sample code declared Account.setAnnualInterestRate instead of account.setAnnualInterestRate.
