Object Oriented style programming for interaction between objects

Question

I am trying to write a program in object-oriented style. I have some confusions when coding the interaction between two objects.

Scenario: Person (John) gives Person (Betty) $ 5.

Possible solutions (pseudo code):

A) John.pays(Betty, 5);
B) Betty.receives(John, 5);
C) Bank.transfer(John, Betty, 5);
D)
begin transaction:
John.decrease(5);
Betty.increase(5);
end transaction:
E) Service.transferMoney(John, Betty, 5); // Service is a generic service object

Please tell me which one is a more appropriate way of coding in OOP way, and the reason behind it. I am looking for some guidelines like "Tell, Don't Ask" rule.

Thanks.

Answer 1

One thing I've noticed is that people that are new to OOP get caught up in trying to map the physical world into the code they are writing. Do you really care that John and Betty are people or are you actually wanting to depict a bank account? I think your choice of objects in the example actually make it harder to figure out the solution to the problem.

The important parts of this are 1) Where to put the logic of how to move the money. 2) Where to store the data of how much money each person has.

You need to decide if you want to talk about the problem in the context of a person or a customer of a bank (may be a person, company, or something else). I'm guessing you are talking about a customer because assuming it is a person would be limiting and misleading. Also, a Bank is a pretty generic term, is it the big brick building with people inside of it or is it the online website with several different pages that do different things. A bank account object can have a method (possibly static depending on how you decide to store your data and what all you are going to use your object for) that knows how to transfer from one account to another. The logic of how to transfer does not belong to Betty or John or a bank, it belongs to a bankAccount which can have special logic based on the type of account if there are fee's involved or the like. If you gave that logic to the bank you would end up with a giant bank class with methods for everything from greating a customer to dealing with money in very specific account types. Each account type my have different rules for how it handles transfers. Think of times where you may want to show a transfer or deposit as pending.

If you are just solving the problem of transfering money, there is no need to create a bunch of objects. Based on the known requirements and presumed future requirements the below would be a good choice. CheckingAccount.Transfer(johnsAccountNo, bettysAccountNo, amount)

Answer 2

Can I ask a question now? Who controls the money? Does John decide the transaction amount, does Betty, or some unspecified 3rd party?

The reason I am asking is because there is no real right or wrong answer here, just one that might be more flexible, or robust than the others. If this is a real life situation then I would model the transaction as something that both parties have to agree on before it proceeds, and the person spending the money (John) initiating it. Something like answer C and @Mysterie Man

tx transaction_request = John.WantsToBuyFor(5);    //check if John can
if( Betty.AgreesWith( transaction_request ) )      //check if Betty wants
{
   transaction_request.FinalizeWith(Betty);        //Do it with Betty
}

and the FinalizeWith function does the math

void FinalizeWith(Person party)
{
    requestor.cash -= amount;
    party.cash += amount;
{

Of course you might want to add some description of what item is John buying.