Inheritance - Data Structures and Problem Solving Using Java - page 128

figure 4.13
A sample
program that
uses the
shape
hierarchy

1 class ShapeDemo
2 {
3 public static double totalArea( Shape [ ] arr )
4 {
5 double total = 0;
6
7 for( Shape s : arr )
8 if( s != null )
9 total += s.area( );
10
11 return total;
12 }
13
14 public static void printAll( Shape [ ] arr )
15 {
16 for( Shape s : arr )
17 System.out.println( s );
18 }
19
20 public static void main( String [ ] args )
21 {
22 Shape [ ] a = { new Circle( 2.0 ), new Rectangle( 1.0, 3.0 ), null };
23
24 System.out.println( "Total area = " + totalArea( a ) );
25 printAll( a );
26 }
27 }

A better solution for area is to throw a runtime exception (a good one is
UnsupportedOperationException ) in the Shape class. This is preferable to return-
ing -1 because the exception will not be ignored.
However, even that solution resolves the problem at runtime. It would be
better to have syntax that explicitly states that area is a placeholder and does
not need any implementation at all, and that further, Shape is a placeholder
class and cannot be constructed, even though it may declare constructors, and
will have a default constructor if none are declared. If this syntax were available,
then the compiler could, at compile time, declare as illegal any attempts to
construct a Shape instance. It could also declare as illegal any classes, such as
Triangle , for which there are attempts to construct instances, even though
area has not been overridden. This exactly describes abstract methods and
abstract classes.

Abstract methods
and classes repre-
sent placeholders.

An abstract method is a method that declares functionality that all derived
class objects must eventually implement. In other words, it says what these
objects can do. However, it does not provide a default implementation.
Instead, each object must provide its own implementation.

An abstract method
has no meaningful
definition and is
thus always defined
in the derived class.