The collections api - Data Structures and Problem Solving Using Java - page 243

6.4.1 Comparator function objects

Many Collections API classes and routines require the ability to order objects.
There are two ways to do this. One possibility is that the objects implement the
Comparable interface and provide a compareTo method. The other possibility is
that the comparison function is embedded as the compare method in an object that
implements the Comparator interface. Comparator is defined in java.util ; a sam-
ple implementation was shown in Figure 4.39 and is repeated in Figure 6.12.

6.4.2 the Collections class

Although we will not make use of the Collections class in this text, it has two
methods that are thematic of how generic algorithms for the Collections API
are written. We write these methods in the Collections class implementation
that spans Figures 6.13 and 6.14.

Figure 6.13 begins by illustrating the common technique of declaring a
private constructor in classes that contain only static methods. This prevents
instantiation of the class. It continues by providing the reverseOrder method.
This is a factory method that returns a Comparator that provides the reverse
of the natural ordering for Comparable objects. The returned object, created at
line 20, is an instance of the ReverseComparator class written in lines 23 to
29. In the ReverseComparator class, we use the compareTo method. This is an

reverseOrder is a
factory method
that creates a
Comparator repre-
senting the
reverse natural
order.

1 package weiss.util;
2
3 /**
4 * Comparator function object interface.
5 */
6 public interface Comparator<AnyType>
7 {
8 /**
9 * Return the result of comparing lhs and rhs.
10 * @param lhs first object.
11 * @param rhs second object.
12 * @return < 0 if lhs is less than rhs,
13 * 0 if lhs is equal to rhs,
14 * > 0 if lhs is greater than rhs.
15 * @throws ClassCastException if objects cannot be compared.
16 */
17 int compare( AnyType lhs, AnyType rhs ) throws ClassCastException;
18 }

figure 6.12
The Comparator interface, originally defined in java.util and rewritten for the weiss.util package