Advanced Data Structures - Java Concepts - page 1003

requirement that you saw in Section 14.8 for using the sort and binarySearch
methods in the standard library.

To use a tree set, the elements must be comparable.

To use a TreeMap , the same requirement holds for the keys. There is no requirement
for the values.

For example, the String class implements the Comparable interface. The
compareTo method compares strings in dictionary order. Thus, you can form tree
sets of strings, and use strings as keys for tree maps.

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If the class of the tree set elements doesn't implement the Comparable interface, or
the sort order of the compareTo method isn't the one you want, then you can define
your own comparison by supplying a Comparator object to the TreeSet or
TreeMap constructor. For example,

Comparator comp = new CoinComparator();
Set s = new TreeSet(comp);

As described in Advanced Topic 14.5 , a Comparator object compares two
elements and returns a negative integer if the first is less than the second, zero if they
are identical, and a positive value otherwise. The example program at the end of this
section constructs a TreeSet of Coin objects, using the coin comparator of
Advanced Topic 14.5 .

ch16/treeset/TreeSetTester.java

1 import java.util.Comparator;
2 import java.util.Set;
3 import java.util.TreeSet;
4
5 /**
6 A program to a test a tree set with a comparator for coins.
7 */
8 public class TreeSetTester
9 {
10 public static void main(String[] args)
11 {
12 Coin coin1 = new Coin( 0.25 , ÐquarterÑ );