ABSTRACT CLASSES AND INTERFACES - Introduction to Java Programming: Comprehensive Version - page 498

43
44 @Override
45

public String toString() {

46 return "created on " + dateCreated + "\ncolor: " + color +

47 " and filled: " + filled;

48 }
49
50 /** Abstract method getArea */

51 public abstract double getArea(); abstract method

52
53 /** Abstract method getPerimeter */

abstract method

54 public abstract double getPerimeter();

55 }

Abstract classes are like regular classes, but you cannot create instances of abstract classes
using the new operator. An abstract method is defined without implementation. Its implemen-
tation is provided by the subclasses. A class that contains abstract methods must be defined
as abstract.
The constructor in the abstract class is defined as protected, because it is used only by
subclasses. When you create an instance of a concrete subclass, its superclass's constructor is
invoked to initialize data fields defined in the superclass.
The GeometricObject abstract class defines the common features (data and methods)
for geometric objects and provides appropriate constructors. Because you don't know how to
compute areas and perimeters of geometric objects, getArea() and getPerimeter() are
defined as abstract methods. These methods are implemented in the subclasses. The imple-
mentation of Circle and Rectangle is the same as in Listings 13.2 and 13.3, except that
they extend the GeometricObject class defined in this chapter. You can see the complete
code for these two programs from www.cs.armstrong.edu/liang/intro10e/html/Circle.html
and www.cs.armstrong.edu/liang/intro10e/html/Rectangle.html, respectively.

why protected constructor?

implement Circle
implement Rectangle

L ISTING 13.2

Circle.java
1 public class Circle extends GeometricObject {
2

extends abstract
GeometricObject

// Same as lines 3-48 in Listing 11.2, so omitted

3 }

L ISTING 13.3

Rectangle.java
1 public class Rectangle extends GeometricObject {
2

extends abstract
GeometricObject

// Same as lines 3-51 in Listing 11.3, so omitted

3 }

13.2.1 Why Abstract Methods?
You may be wondering what advantage is gained by defining the methods getArea() and
getPerimeter() as abstract in the GeometricObject class. The example in Listing 13.4
shows the benefits of defining them in the GeometricObject class. The program creates two
geometric objects, a circle and a rectangle, invokes the equalArea method to check whether
they have equal areas, and invokes the displayGeometricObject method to display them.

L ISTING 13.4

TestGeometricObject.java
1 public class TestGeometricObject {
2 /** Main method */

3 public static void main(String[] args) {

4 // Create two geometric objects

5 GeometricObject geoObject1 = new Circle( 5 ); create a circle

create a rectangle 6 GeometricObject geoObject2 = new Rectangle( 5 , 3 );