Subclasses - Multimedia Introduction to Programming Using Java - page 163

This kind of situation occurs frequently, so Java provides a construct, the
abstract class , to handle it better. We use class Shape of Fig. 4.9 to illustrate. It
is written as an abstract class in Fig. 4.14. It differs from Shape of Fig. 4.9 in two
ways.

1. The class has been changed into an abstract class. This is done by insert-
ing keyword abstract before keyword class in the first line of the class
definition:

public abstract class Shape {
An abstract class cannot be instantiated: expression new Shape(…) is ille-
gal.

2. Method drawShape has been made into an abstract method . This is done
by inserting keyword abstract in the method definition and replacing
the body by a semicolon:

public abstract void drawShape ( … );
An abstract method of an abstract class must be overridden in every sub-
class (unless the subclass is also abstract).

That is all there is to abstract classes and abstract methods: an abstract class
cannot be instantiated, and an abstract method must be overridden. With these

import java.awt.*;

/** A shape on a screen. */
public abstract class Shape {

/** Constructor: a shape that fits in a bounding rectangle with upper-left corner (x, y) */
public Shape ( int x, int y){ }

/** = x-coordinate of upper-left corner of bounding rectangle */
public int getX()
{ return 0; }

/** = x-coordinate of upper-left corner of bounding rectangle */
public int getX()
{ return 0; }

/** = a description of this shape, of the form ( x-coordinate , y-coordinate ) */
public String toString()
{ return ""; }

/** Draw this shape using g*/
public abstract void drawShape(Graphics g);

}

Figure 4.14: Design of class Shape as an abstract class, with abstract method drawShape