OO Basics: Create the right object

Most people learn structured programming before they are exposed to object oriented programming. That's just the nature of the beast, since you need to learn the basics of a language before you can move on. At least that's the latest theory. The problem with structured programming is you learn to use flags to signal changes in state. You then end up with ifs and switches all over the code. For example

if (item.isTaxable()) {
  item.setCost(item.getCost *= TAX_RATE);
}

The root of the problem is that you only have one kind of Item.

public class Item {
  private boolean taxable;

  public boolean isTaxable() {
    return taxable;
  }
  ...
}

where two types are needed:

public interface Item {
  double getTax();
}

public class TaxableItem implements Item {
  public double getTax() {
    return cost * TAX_RATE;
  }
}

public class NontaxableItem implements Item {
  public double getTax() {
    return 0;
  }
}

You also have a problem with how the object is created:

Item item = new Item(true);

Say you are implementing a shopping cart, and as the user adds items to the cart, the correct type of item is added. Then at the end, you can calculate tax for the taxable items because the cart already contains the right kind of item.

cart.addItem(ItemFactory.createItem(itemId));

Now there are no ifs in the domain code:

for (Item item : cart.items()) {
    tax += item.getTax();
}

The moral: a lot of ifs and their accompanying headaches go away when you take the effort to create the right object.