Polymorphism and Abstract Classes - Absolute Java - page 510

Abstract Classes
In Chapter 7 , we defined a class named Employee and two of its derived classes,
HourlyEmployee and SalariedEmployee . Display 8.6 repeats the details of these
class definitions, which we will use in this discussion.
Suppose that when we define the class Employee , we know that we are going to
frequently compare employees to see if they have the same pay. We might add the
following method to the class Employee :

public boolean samePay(Employee other)
{

return ( this .getPay() == other.getPay());

}

There is, however, one problem with adding the method samePay to the class
Employee : The method samePay includes an invocation of the method getPay , and
the class Employee has no getPay method. Moreover, there is no reasonable definition
we might give for a getPay method so that we could add it to the class Employee .
The only instance variables in the class Employee give an employee's name and hire
date, but give no information about pay. To see how we should proceed, let's compare
objects of the class Employee to employees in the real world.
Every real-world employee does have some pay because every real-world employee
is either an hourly employee or a salaried employee, and the two derived classes
HourlyEmployee and SalariedEmployee each have a getPay method. The problem is
that we do not know how to define the getPay method until we know if the employee is
an hourly or salaried. We would like to postpone the definition of the getPay method and
give it only in each derived class of the Employee class. We would like to simply add a note
to the Employee class that says: “There will be a method getPay for each Employee but
we do not yet know how it is defined.” Java lets us do exactly what we want. The official
Java equivalent of our promissory note about the method getPay is to make getPay an
abstract method . An abstract method has a heading just like an ordinary method, but
no method body. The syntax rules of Java require the modifier abstract and require a
semicolon in place of the missing method body, as illustrated by the following:

abstract
method

public abstract double getPay();