When should a subclass have its own fields and when should it use its super

Question

When should a subclass have its own fields and when should it use its superclass' fields?
Hi, thank you for reading this post!
Let me use a specific example to ask my question.
public class BankAccount {
     private double accountBalance;
     public double getBalance() {
          return this.accountBalance;
public class SavingsAccount extends BankAccount {
     private double accountBalance;
     public double getBalance() {
          return this.accountBalance;
}In the bank account example, both BankAccount and SavingsAccount will have a method getBalance(). Therefore, they
both require a account balance field. My question is since getBalance() for both classes will perform the exact same
operation, when should I omit declaring the getBalance() method and the accountBalance field in the subclass, and
when should I include them?
My own thought is when we never have to instantiate a superclass object (e.g. an abstract class), then we place
common fields in the abstract superclass and have subclasses access these fields via protected getter/setters to
access the superclass' fields. This is the principle of reuse.
But when you do need to instantiate a superclass and the superclass does need to maintain its own fields, then
I would need to duplicate the accountBalance field and getBalance() method in the subclass.
Is my thinking correct or incorrect?
Thank you in advance for your help!
Eric
Edited by: er**** on 22-Aug-2011 20:19

Answer 1

er**** wrote:
If SavingsAccount inherit BankAccount.getBalance()...getBalance() would return BankAccount's accountBalance. This is NOT the correct result we want.Actually, I think it's precisely what you want.
We want getBalance() to return BankAccount's accountBalance when we use a BankAccount object, and SavingsAccount's accountBalance when we use a SavingsAccount object.I seriously doubt that. I think you're confusing a BankAccount with a Customer, who can have more than one account.
In every system I've ever seen, a SavingsAccount IS-A BankAccount - that is to say, it's a genuine subtype. Now, it may well contain other fields ('interest'?) that a normal account wouldn't, but 'balance' ain't one of them.
Winston