Example Problem

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

• Goods, each of which has the attributes:
  • description
  • price
• The types of goods are:
  • Food—with an attribute "calories". Food objects are not taxable.
  • Toy—with an attribute "minimumAge". Toy objects are taxable.
  • Book—with an attribute "author". Book objects are taxable.

There are many things that are taxable that are not goods, such as services or entertainment. Also, not all goods are taxable. 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.