ABSTRACT CLASSES AND INTERFACES - Introduction to Java Programming: Comprehensive Version - page 511

Atlanta Boston Savannah Tampa
54623239292 432232323239292 2323231092923992

The program creates an array of strings (line 5) and invokes the sort method to sort
the strings (line 6). The program creates an array of BigInteger objects (lines 11-13) and
invokes the sort method to sort the BigInteger objects (line 14).
You cannot use the sort method to sort an array of Rectangle objects, because
Rectangle does not implement Comparable . However, you can define a new rectangle
class that implements Comparable . The instances of this new class are comparable. Let this
new class be named ComparableRectangle , as shown in Listing 13.9.

L ISTING 13.9 ComparableRectangle.java

1 public class ComparableRectangle extends Rectangle
2 implements Comparable<ComparableRectangle> {

implements Comparable

3 /** Construct a ComparableRectangle with specified properties */

4 public ComparableRectangle( double width, double height) {

5 super (width, height);

6 }
7
8 @Override // Implement the compareTo method defined in Comparable
9

public int compareTo(ComparableRectangle o) {

implement compareTo

10 if (getArea() > o.getArea())

11 return 1 ;

12 else if (getArea() < o.getArea())

13 return -1 ;

14 else

15 return 0 ;

16 }
17
18 @Override // Implement the toString method in GeometricObject
19

public String toString() {

implement toString

20 return super .toString() + " Area: " + getArea();

21 }
22 }

ComparableRectangle extends Rectangle and implements Comparable , as shown
in Figure 13.5. The keyword implements indicates that ComparableRectangle inher-
its all the constants from the Comparable interface and implements the methods in the
interface. The compareTo method compares the areas of two rectangles. An instance of
ComparableRectangle is also an instance of Rectangle , GeometricObject , Object ,
and Comparable .

GeometricObject «interface»

java.lang.Comparable<ComparableRectangle>

+compareTo(o: ComparableRectangle): int

Rectangle

ComparableRectangle

F IGURE 13.5

ComparableRectangle extends Rectangle and implements Comparable .