LISTS, STACKS, QUEUES, AND PRIORITY QUEUES - Introduction to Java Programming: Comprehensive Version - page 773

5 GeometricObject g1 = new Rectangle( 5 , 5 );
6 GeometricObject g2 = new Circle( 5 );
7
8 GeometricObject g =
9 max(g1, g2, new GeometricObjectComparator()); invoke max

10
11 System.out.println( "The area of the larger object is " +
12 g.getArea());
13 }
14
15 public static GeometricObject max(GeometricObject g1,
16 GeometricObject g2, Comparator<GeometricObject> c) {
17

the max method

if (c.compare(g1, g2) > 0 )

invoke compare

18 return g1;

19 else

20 return g2;

21 }
22 }

The area of the larger object is 78.53981633974483

The program creates a Rectangle and a Circle object in lines 5-6 (the Rectangle and
Circle classes were defined in Section 13.2, Abstract Classes). They are all subclasses of
GeometricObject . The program invokes the max method to obtain the geometric object
with the larger area (lines 8-9).
The GeometricObjectComparator is created and passed to the max method (line 9) and
this comparator is used in the max method to compare the geometric objects in line 17.

Note
Comparable is used to compare the objects of the class that implement Comparable .
Comparator can be used to compare the objects of a class that doesn't implement
Comparable .
Comparing elements using the Comparable interface is referred to as comparing using
natural order , and comparing elements using the Comparator interface is referred to
as comparing using comparator .

Comparable vs. Comparator

natural order
using comparator

20.15

✓ ✓

What are the differences between the Comparable interface and the Comparator
interface? In which package is Comparable , and in which package is Comparator ?

Check

Point

20.16

How do you define a class A that implements the Comparable interface? Are two
instances of class A comparable? How do you define a class B that implements the
Comparator interface and override the compare method to compare to objects of
type B1 ? How do you invoke the sort method to sort a list of objects of the type B1 ?

20.6 Static Methods for Lists and Collections
The Collections class contains static methods to perform common operations in a
collection and a list.

Key
Point

Section 11.12 introduced several static methods in the Collections class for array lists.
The Collections class contains the sort , binarySearch , reverse , shuffle , copy , and
fill methods for lists, and max , min , disjoint , and frequency methods for collections,
as shown in Figure 20.7.