Example Problem

Let us create a database program for a store. The store sells:

• Goods, each of which has the attributes:
  • description
  • price
• The goods are either:
  • Food—with an attribute "calories"
  • Toy—with an attribute "minimumAge", or
  • Book—with an attribute "author."

Of these goods, toys and books are taxable, but food is not. There are many other things that are taxable, such as services or entertainment so we want to have the concept "taxable" as a separate concept, not part of the concept of Goods.

Here is what the concept Taxable looks like:

• A Taxable item,
  • has a taxRate of 6 percent,
  • has a calculateTax() method.

When implemented in Java, these concepts will appear as classes and an interface.