LISTS, STACKS, QUEUES, AND PRIORITY QUEUES - Introduction to Java Programming: Comprehensive Version - page 772

20.5 The Comparator Interface
Comparator can be used to compare the objects of a class that doesn't implement
Comparable .

Key
Point

You have learned how to compare elements using the Comparable interface (introduced
in Section 13.6). Several classes in the Java API, such as String , Date , Calendar ,
BigInteger , BigDecimal , and all the numeric wrapper classes for the primitive types,
implement the Comparable interface. The Comparable interface defines the compareTo
method, which is used to compare two elements of the same class that implement the
Comparable interface.
What if the elements' classes do not implement the Comparable interface? Can these
elements be compared? You can define a comparator to compare the elements of different
classes. To do so, define a class that implements the java.util.Comparator<T> interface
and overrides its compare method.

comparator

public int compare(T element1, T element2)

Returns a negative value if element1 is less than element2 , a positive value if
element1 is greater than element2 , and zero if they are equal.

The GeometricObject class was introduced in Section 13.2, Abstract Classes. The
GeometricObject class does not implement the Comparable interface. To compare the
objects of the GeometricObject class, you can define a comparator class, as shown in
Listing 20.4.

L ISTING 20.4

GeometricObjectComparator.java
1 import java.util.Comparator;
2
3 public class GeometricObjectComparator
4 implements Comparator<GeometricObject>, java.io.Serializable { implements Comparator

implements compare 5 public int compare(GeometricObject o1, GeometricObject o2) {

6 double area1 = o1.getArea();

7 double area2 = o2.getArea();

8
9 if (area1 < area2)

10 return -1 ;

11 else if (area1 == area2)

12 return 0 ;

13 else

14 return 1 ;

15 }
16 }

Line 4 implements Comparator<GeometricObject> . Line 5 overrides the compare
method to compare two geometric objects. The class also implements Serializable . It is
generally a good idea for comparators to implement Serializable , as they may be used
as ordering methods in serializable data structures. In order for the data structure to serialize
successfully, the comparator (if provided) must implement Serializable .
Listing 20.5 gives a method that returns a larger object between two geometric objects. The
objects are compared using the GeometricObjectComparator .

L ISTING 20.5

TestComparator.java
1 import java.util.Comparator;
2
3 public class TestComparator {
4 public static void main(String[] args) {