SETS AND MAPS - Introduction to Java Programming: Comprehensive Version - page 803

a portion of the set whose elements are less than toElement and greater than or equal to
fromElement , respectively.
NavigableSet extends SortedSet to provide navigation methods lower(e) ,
floor(e) , ceiling(e) , and higher(e) that return elements respectively less than, less
than or equal, greater than or equal, and greater than a given element and return null if there
is no such element. The pollFirst() and pollLast() methods remove and return the first
and last element in the tree set, respectively.
TreeSet implements the SortedSet interface. To create a TreeSet , use a constructor,
as shown in Figure 21.1. You can add objects into a tree set as long as they can be compared
with each other.
As discussed in Section 20.5, the elements can be compared in two ways: using the
Comparable interface or the Comparator interface.
Listing 21.4 gives an example of ordering elements using the Comparable interface. The
preceding example in Listing 21.3 displays all the strings in their insertion order. This example
rewrites the preceding example to display the strings in alphabetical order using the TreeSet class.

tree set

L ISTING 21.4

TestTreeSet.java
1 import java.util.*;
2
3 public class TestTreeSet {
4

public static void main(String[] args) {

5 // Create a hash set

6 Set<String> set = new HashSet<>(); create hash set

7
8 // Add strings to the set
9 set.add( "London" );
10 set.add( "Paris" );
11 set.add( "New York" );
12 set.add( "San Francisco" );
13 set.add( "Beijing" );
14 set.add( "New York" );
15
16 TreeSet<String> treeSet = new TreeSet<>(set);
17 System.out.println( "Sorted tree set: " + treeSet);
18
19 // Use the methods in SortedSet interface
20 System.out.println( "first(): " + treeSet.first());
21 System.out.println( "last(): " + treeSet.last());
22 System.out.println( "headSet(\"New York\"): " +
23 treeSet.headSet( "New York" ));
24 System.out.println( "tailSet(\"New York\"): " +
25 treeSet.tailSet( "New York" ));
26
27 // Use the methods in NavigableSet interface
28 System.out.println( "lower(\"P\"): " + treeSet.lower( "P" ));
29 System.out.println( "higher(\"P\"): " + treeSet.higher( "P" ));
30 System.out.println( "floor(\"P\"): " + treeSet.floor( "P" ));
31 System.out.println( "ceiling(\"P\"): " + treeSet.ceiling( "P" ));
32 System.out.println( "pollFirst(): " + treeSet.pollFirst());
33 System.out.println( "pollLast(): " + treeSet.pollLast());
34 System.out.println( "New tree set: " + treeSet);
35 }
36 }

create tree set

display elements

Sorted tree set: [Beijing, London, New York, Paris, San Francisco]
first(): Beijing
last(): San Francisco