Should I instantiate a collection or inherit from collection?

Question

I've asked myself this question a number of times when creating classes, particularly those involving collections, but I've never come up with a satisfactory answer. It's a OOP design question.

For example, in a checkbook register program say I have a class of BankAccount. BankAccounts contain data involving the Name of the account, the Type of account (enum of Checking, Saving,...), and other data, but most importantly is a collection of the Adjustments (deposits or withdrawals) in the account.

Here, I have two options for keeping a collection of the Adjustments:

1. Instantiate a collection and keep it as a member within the BankAccount class. This is like saying "BankAccount has a collection of Adjustments."
2. Inherit from collection. This is like saying "BankAccount is a collection of Adjustments."

I think both solutions are intuitive, and, of course, both offer some advantages and disadvantages. For example, instantiating allows the class (in languages that allow only a single base class) to inherit from another class, while inheriting from collection makes it easy to control the Add, Remove, and other methods without having to write surrogate methods to 'wrap' those.

So, in such situations, which is a better approach?

Answer 1

To me, a bank account has a collection of adjustments. A bank account is not a collection of adjustments, because it "is" much more than that: it also "is" a name and a type, etc.

So, in your case (and similar cases), I suggest you aggregate a collection inside your class.

In case of doubt, avoid inheritance. :-)

I can argument this further. In order to use inheritance properly, the subclass must satisfy Liskov's substitution principle; this means that, in your case, BankAccount should be a valid type anywhere a Collection is expected. I don't think that's the case, because a Collection probably exposes methods such as Add() and Remove(), whereas you will want to exert some control over adding and removing adjustments from your bank account rather than letting people add and remove them freely.