A stamp has a descriptive name, a face value, and an issuing country. Stamps have a measurable value (the face value), hence it makes sense for the Stamp class to implement the Measurable interface from Chapter 9. You can also compare two stamps to find which has higher value, so it makes sense to implement the Comparable interface from the standard Java library as well.

In fact, any two measurable objects can be compared, simply by comparing their measures. We can implement the compareTo method of the Comparable interface in a class AbstractMeasurable that implements both the Comparable and Measurable interfaces. Then the Stamp class simply extends that class.