Question

public class OrderProcessor {

  public Double calculateTotalPriceWithDiscountCode(Order order,
        char discountCode) {

    Double itemTotal = order.getItemTotal();
    Double discountAmount = 0.0;

    switch (discountCode) {
    case 'A':
        discountAmount = 0.95 * itemTotal;
        break;
    case 'B':
        discountAmount = 0.15 * itemTotal;
        break;
    }
    return itemTotal - discountAmount;
}

Current implementation in order processor is closed for extension and open for modification for adding new discount codes, how can i improve the design to get rid of this limitation

Answer 1

In general, the presence of a switch is a pretty good giveaway as to what should be a class instead. So, a first stab at it would be something like this:

public interface Discounter {
  public double applyDiscount(double itemTotal);
}

public class OrderProcessor {
  private Map<Char, Discounter> discounts = new HashMap<Char, Discounter>();

  public void addDiscounter(Char discountCode, Discounter discounter) {
    discounts.put(discountCode, discounter);
  }

  public Double calculateTotalPriceWithDiscountCode(Order order, char discountCode) {
    double itemTotal = order.getItemTotal();
    double discountAmount = 0.0;

    if (discounts.hasKey(discountCode))
      discountAmount = discounter.applyDiscount(itemTotal);

    return itemTotal - discountAmount;
  }
}

This would then be extended through something like this:

processor.addDiscounter('A', new Discounter() {
  public double applyDiscount(double itemTotal) {
    return 0.95 * itemTotal;
  }
});

You could make a remove method as well, and then your discounters can become more complex, consulting external data, etc. You may want to open up the interface a bit and pass in the whole order for more examination.

Note: If this is something you’re going to be doing in production I have two pieces of unsolicited advice:

1. Consider using something like JBoss Drools to handle your business logic instead of this; it’s a lot more powerful and flexible.

2. Please don’t use double for actual financial calculations. 🙂