Account acct2 = new SavingsAccount (name); acct2.calculateBalance();
This is because although you have an object of SavingsAccount you are using refrence variable of type Account so you can access only those methods that are there in Account class.
And you don’t have calculateBalance() method in your Account class.
That’s why you are not able to access it and compiler complains that it cannot find a method named calculateBalance as it sees that reference type is Account and there is no such method inside Account class.
If you want to use that method then change reference type to SavingsAccount :
SavingsAccount acct2 = new SavingsAccount (name);
Or you can explicitly cast it when accessing that method
((SavingsAccount) acct2).calculateBalance();
but be alert that it can throw ClassCastException if acct2 object is actually not an object of SavingsAccount
UPDATE:
But remember that at runtime, Java uses virtual method invocation to dynamically select the actual version of the method that will run, based on the actual instance.