13.2 Abstract Classes
13.2 Abstract Classes
The inheritance relationship was discussed in Chapter 12. Figure 13.1 illustrates a simple inheritance relationship with four classes Gfigures, Triangle, Circle, and Rectangle.
Figure 13.1: A generic base class.
The characteristics of the geometric figures are related in some way. Assume that the relevant attributes defined in these classes are height, base, and radius. The relevant functions defined in the classes are area and perimeter.
An abstract class is one that includes one or more abstract methods. An abstract method has no implementation, only its declaration (also called its specification).
| Caution | An abstract class cannot be instantiated; it is generally used as a base class. The subclasses override the abstract methods inherited from the abstract base class. |
13.2.1 Defining an Abstract Class
To define an abstract class, the keyword abstract should be used before the keyword class. Every function that does not include its implementation is also preceded by the keyword abstract. An abstract class definition in KJP has the following general structure:
description . . . abstract class 〈 class_name 〉 is private // private attributes constants . . . variables . . . objects . . . // private operations . . . public // public operations . . . endclass 〈 class_name 〉
The attributes of derived classes Rectangle, Triangle, and Circle are different. Classes Rectangle and Triangle have the attributes height and base; Class Circle has the attribute radius. The calculations of area and perimeter are different in each of these subclasses. In this situation, the base class (Gfigures) cannot include the implementations for functions area and perimeter. It can only provide the prototypes for these functions.
A base class is an abstract class when it does not provide the implementation for one or more functions. Such base classes provide single general descriptions for the common functionality and structure of its subclasses. An abstract class is a foundation on which to define subclasses. Classes that are not abstract classes are known as concrete classes.
The base class Gfigures is an abstract class because it does not provide the implementation of the functions area and perimeter. The KJP code with the definition of class Gfigures follows.
description This abstract class has two functions. */ abstract class Gfigures is public description This function computes and returns the area of the geometric figure. */ abstract function area of type double description This function computes and returns the perimeter of the geometric figure. */ abstract function perimeter of type double endclass Gfigures
| On the CD | The KJP code with the implementation of this class is stored in the file |
Gfigures.kpl| Caution | Because the abstract class Gfigures does not include the body of the functions area and perimeter, objects of this class cannot be created. In other words, class Gfigures cannot be instantiated. |
In Java, the structure of the abstract class is very similar. The definition of this abstract class follows.
// KJP v 1.1 File: Gfigures.java, Sat Jan 04 19:03:07 2003 /** This abstract class with two functions. */ public abstract class Gfigures { /** This function computes and returns the area of the geometric figure. */ abstract public double area(); /** This function computes and returns the perimeter of the geometric figure. */ abstract public double perimeter(); } // end class Gfigures
13.2.2 Using Abstract Classes
The subclasses that inherit an abstract base class need to override (redefine) the functions that are defined as abstract functions in the abstract base class. The subclasses that are shown in Figure 13.1, Rectangle, Triangle, and Circle, inherit class Gfigures and each includes the specific implementation of the functions area and perimeter and the relevant attributes.
| On the CD | The following KJP code includes the complete definition of class Triangle. This inherits the function from class Gfigures and redefines functions area and perimeter. The code for this class is stored in the file |
description This class computes the area and perimeter of a triangle, given its three sides. */ class Triangle inherits Gfigures is private variables double x // first side double y // second side double z // third side public description This constructor sets values for the three sides of the triangle. */ function initializer parameters double i_x, double i_y, double i_z is begin set x = i_x set y = i_y set z = i_z endfun initializer description This function computes the perimeter of a triangle. */ function perimeter of type double is variables double lperim begin set lperim = x + y + z return lperim endfun perim description This function computes the area of a triangle. */ function area of type double is variables double s // intermediate result double r double area begin set s = 0.5 * (x + y + z) set r = s * (s - x)*(s - y)*(s - z) set area = Math.sqrt(r) return area endfun area endclass Triangle
To construct a complete application with class Triangle, a new class named Mtriangle is defined. This class includes function main that reads the three values for the sides of the triangle. Then, it creates an object of class Triangle and invokes functions area and perimeter of the object.
description This program computes the area and peri- meter of a triangle, given its three sides. */ class Mtriangle is public description This function gets the area and perimeter of a triangle object. */ function main is variables double x // first side double y // second side double z // third side double area double perim objects object my_triangle of class Triangle begin display "This program computes the area" display " of a triangle" display "enter value of first side: " read x display "enter value of second side: " read y display "enter value of third side: " read z create my_triangle of class Triangle using x, y, z set area = call area of my_triangle display "Area of triangle is: ", area set perim = call perimeter of my_triangle display "Perimeter of triangle is: ", perim endfun main endclass Mtriangle
After translating the classes Triangle and Mtriangle and compiling them with the Java compiler, class Mtriangle can be executed. The output produced on the console is:
This program computes the area of a triangle enter value of first side: 2 enter value of second side: 4 enter value of third side: 5 Area of triangle is: 3.799671038392666 Perimeter of triangle is: 11.0
EAN: 2147483647
Pages: 184