ABSTRACT CLASSES AND INTERFACES - Introduction to Java Programming: Comprehensive Version - page 512

You can now use the sort method to sort an array of ComparableRectangle objects,
as in Listing 13.10.

L ISTING 13.10

SortRectangles.java
1 public class SortRectangles {
2 public static void main(String[] args) {
3 ComparableRectangle[] rectangles = {
4 new ComparableRectangle( 3.4 , 5.4 ),
5 new ComparableRectangle( 13.24 , 55.4 ),
6 new ComparableRectangle( 7.4 , 35.4 ),
7 new ComparableRectangle( 1.4 , 25.4 )};
8 java.util.Arrays.sort(rectangles);
9 for (Rectangle rectangle: rectangles) {
10 System.out.print(rectangle + " " );
11 System.out.println();
12 }
13 }
14 }

create an array

sort the array

An interface provides another form of generic programming. It would be difficult to use a
generic sort method to sort the objects without using an interface in this example, because

Width: 3.4 Height: 5.4 Area: 18.36
Width: 1.4 Height: 25.4 Area: 35.559999999999995
Width: 7.4 Height: 35.4 Area: 261.96
Width: 13.24 Height: 55.4 Area: 733.496

benefits of interface

multiple inheritance would be necessary to inherit Comparable and another class, such as
Rectangle , at the same time.
The Object class contains the equals method, which is intended for the subclasses of the
Object class to override in order to compare whether the contents of the objects are the same.
Suppose that the Object class contains the compareTo method, as defined in the Comparable
interface; the sort method can be used to compare a list of any objects. Whether a compareTo
method should be included in the Object class is debatable. Since the compareTo method is
not defined in the Object class, the Comparable interface is defined in Java to enable objects
to be compared if they are instances of the Comparable interface. It is strongly recommended
(though not required) that compareTo should be consistent with equals . That is, for two
objects o1 and o2 , o1.compareTo(o2) == 0 if and only if o1.equals(o2) is true .

13.17

✓ ✓

True or false? If a class implements Comparable , the object of the class can invoke
the compareTo method.

Check

Point

13.18

Which of the following is the correct method header for the compareTo method in
the String class?

public int compareTo(String o)
public int compareTo(Object o)

13.19

Can the following code be compiled? Why?

Integer n1 = new Integer( 3 );
Object n2 = new Integer( 4 );
System.out.println(n1.compareTo(n2));