Object-Oriented Concepts - SCJP Sun Certified Programmer for Java Platform SE6 - page 391

Suppose I need to write a class to represent employees of a company. Because the Pet
class contains fi elds like name and age , I might be tempted to write an Employee class that
extends Pet to reuse the code in Pet , as Figure 6.2 shows.

FIGURE 6.2 Good inheritance design needs to satisfy the “is a” relationship.

Pet

name : String
age : int

eat() : void

Poor design: An employee
is not a pet.

Employee

salary : double
work() : void

Although this design might work functionally and allow me to store an employee's name
and age in the fi elds of Pet , the design is not a good one because an employee is not a pet.
A better design is for the Employee class to extend a class like Person , because most
likely an employee “is a” person. Figure 6.3 shows what these classes might look like.

FIGURE 6.3 An employee is a person, so Employee extending Person is a good design.

Person

name : String
age : int

Good design: An employee
is a person.

Employee

salary : double
work() : void