Question

I have a Chequing account and a Saving account. I’m exploring how to implement the withdraw method using a Strategy pattern.

Currently, the Chequing and Saving account both inherit from Account. For Saving account, withdrawals should not cause the balance to dip below 100 dollars. With the Chequing account, withdrawals must include a cheque number.

I’m not confident in using this approach because, as you’ll see below, the “otherArguments” parameter is totally useless in one scenario. And the only reason I have it like this is to “show” the use of the strategy pattern.

(for those who are concerned, this is part of a school project, all the code below I wrote and I’m curious about if there is a better way to get it done).

Here’s what I’ve done so far:

public abstract class Account
{
    public double Balance{get; set;}

    public WithdrawStrategy Withdrawer
    {
        get; set;
    }


    public abstract void withdraw(double currentBalance, double amount, object otherArguments);
}

public class Chequing: Account
{
    public Chequing()
    {
        Withdrawer= new ChequingAccountWithdrawer();
    }


    public override void withdraw(double currentBalance, double amount, object otherArguments)
    {
        if (null != Withdrawer)
        {
            double balance = Withdrawer.withdraw(currentBalance, amount, otherArguments);
            Balance = balance;
        }
    }    
}

public class Saving: Account
{
    public Saving()
    {
        Withdrawer= new SavingAccountWithdrawer();
    }


    public override void withdraw(double currentBalance, double amount, object otherArguments)
    {
        if (null != Withdrawer)
        {
            double balance = Withdrawer.withdraw(currentBalance, amount, otherArguments);
            Balance = balance;
        }
    }    
}

public interface WithdrawStrategy
{
    double withdraw(double currentBalance, double amount, object otherArguments);
}

public ChequingAccountWithdrawer: WithdrawStrategy
{
    public double withdraw(double currentBalance, double amount, object otherArguments)
    {
        string cheqNum = otherArguments.ToString();
        if (!string.IsNullOrEmpty(cheqNum))
        {
            currentBalance -= amount;
        }
        return currentBalance;
    }
}

public SavingAccountWithdrawer: WithdrawStrategy
{
    public double withdraw(double currentBalance, double amount, object otherArguments)
    {
        if (currentBalance - amount > 100) //hard code for example's sake
        {
            currentBalance -= amount;
        }
        return currentBalance;
    }
}

Answer 1

In using the Strategy pattern, you might have to supply information that is in fact useless. It’s one of the negative consequences. The other solution is to pass in the necessary information into the Strategy object via constructor. This ensures that the Strategy object has the minimal amount of information it requires.

A better solution might be the Template Method though, if the 100 minimum is the only algorithmic difference.

Here’s the sample code.

public abstract class Account
{
    public double Balance{get; set;}
    public abstract void withdraw();
}

public class Chequing: Account
{
    public override void withdraw()
    {
        //It's not clear where the values for your constructor come from, but
        //you get the idea.
        WithdrawStrategy withdrawer= new ChequingAccountWithdrawer();
        /////////////////////////////////////////////////////////////
        double balance = Withdrawer.withdraw();
        Balance = balance;
    }    
}

public class Saving: Account
{
    public override void withdraw()
    {
        //Same idea here.
        WithdrawStrategy withdrawer= new SavingAccountWithdrawer();
        /////////////////////////////////////////////////////////////
        double balance = Withdrawer.withdraw();
        Balance = balance;
    }    
}

public interface WithdrawStrategy
{
    double withdraw();
}

public ChequingAccountWithdrawer: WithdrawStrategy
{
    private readonly int balance;
    private readonly double amount;
    private readonly string number;

    public ChequingAccountWithdrawer(double aBalance, double aAmount, string aNumber) {
        balance = aBalance
        amount = aAmount
        number = aNumber
    }

    public double withdraw()
    {
        if (number)
        {
            currentBalance -= amount;
        }

        return currentBalance;
    }
}

public SavingAccountWithdrawer: WithdrawStrategy
{
    private readonly int balance;
    private readonly double amount;

    public SavingAccountWithdrawer(double aBalance, double aAmount) {
        balance = aBalance
        amount = aAmount
    }

    public double withdraw()
    {
        if (currentBalance - amount > 100) //hard code for example's sake
        {
            currentBalance -= amount;
        }

        return currentBalance;
    }
}

There’s some code duplication. Since it’s homework, there’s not a whole lot you can do. If it was my call though, I’d abstract those fields right out.