Inheritance - Data Structures and Problem Solving Using Java - page 127

figure 4.12
Circle and Rectangle
classes

1 public class Circle extends Shape
2 {
3 public Circle( double rad )
4 { radius = rad; }
5
6 public double area( )
7 { return Math.PI * radius * radius; }
8
9 public double perimeter( )
10 { return 2 * Math.PI * radius; }
11
12 public String toString( )
13 { return "Circle: " + radius; }
14
15 private double radius;
16 }
17
18 public class Rectangle extends Shape
19 {
20 public Rectangle( double len, double wid )
21 { length = len; width = wid; }
22
23 public double area( )
24 { return length * width; }
25
26 public double perimeter( )
27 { return 2 * ( length + width ); }
28
29 public String toString( )
30 { return "Rectangle: " + length + " " + width; }
31
32 public double getLength( )
33 { return length; }
34
35 public double getWidth( )
36 { return width; }
37
38 private double length;
39 private double width;
40 }

The programmer has attempted to signal that calling Shape 's area is an
error by returning -1, which is an obviously impossible area. But this is a
value that might be ignored. Furthermore, this is a value that will be returned
if, when extending Shape , area is not overridden. This failure to override could
occur because of a typographical error: An Area function is written instead of
area , making it difficult to track down the error at run time.