SETS AND MAPS - Introduction to Java Programming: Comprehensive Version - page 802

retains the common elements in set1 . Since set1 and set2 have no common elements,
set1 becomes empty.

21.2.2 LinkedHashSet
LinkedHashSet extends HashSet with a linked-list implementation that supports an order-
ing of the elements in the set. The elements in a HashSet are not ordered, but the elements
in a LinkedHashSet can be retrieved in the order in which they were inserted into the set. A
LinkedHashSet can be created by using one of its four constructors, as shown in Figure 21.1.
These constructors are similar to the constructors for HashSet .
Listing 21.3 gives a test program for LinkedHashSet . The program simply replaces
HashSet by LinkedHashSet in Listing 21.1. linked hash set

L ISTING 21.3

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

public static void main(String[] args) {

5 // Create a hash set

6 Set<String> set = new LinkedHashSet<>();

create linked 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 System.out.println(set);
17
18

add element

// Display the elements in the hash set

19 for (Object element: set) display elements

20 System.out.print(element.toLowerCase() + " " );

21 }
22 }

[London, Paris, New York, San Francisco, Beijing]
london paris new york san francisco beijing

A LinkedHashSet is created in line 6. As shown in the output, the strings are stored in the order
in which they are inserted. Since LinkedHashSet is a set, it does not store duplicate elements.
The LinkedHashSet maintains the order in which the elements are inserted. To impose a
different order (e.g., increasing or decreasing order), you can use the TreeSet class, which is
introduced in the next section.

Tip
If you don't need to maintain the order in which the elements are inserted, use HashSet ,
which is more efficient than LinkedHashSet .

21.2.3 TreeSet
SortedSet is a subinterface of Set , which guarantees that the elements in the set are sorted.
Additionally, it provides the methods first() and last() for returning the first and last
elements in the set, and headSet(toElement) and tailSet(fromElement) for returning