Collections, Maps and Iterators - Absolute Java - page 910

It is important to note that if you intend to use the HashSet<T> class with your own
class as the parameterized type T , then your class must override the following methods:

public int hashCode();
public boolean equals(Object obj);

The hashCode() method should return a numeric key that is ideally a unique
identifier for an object in your class. See Section 15.5 for a discussion about hash codes.
It is always a good idea to override the equals() method for any class you write, but
you must override it in this scenario. Java will use the hash code to index the object and
then use the equals() method to check if an object exists in the set. If the hash code
for two different objects is identical, the objects will still be indexed correctly as long
as equals() indicates they are unique. However, the identical hash codes will cause a
collision that will decrease performance.
Display 16.6 shows a sample program that uses the HashSet<T> class. This
program is conceptually similar to the program in Display 15.38, in which sets
containing strings of round things and green things were manipulated in various

Display 16.6 HashSet<T> Class Demo (part 1 of 3)

1 import java.util.HashSet;
2 import java.util.Iterator;
3 public class HashSetDemo
4 {
5 private static void outputSet(HashSet<String> set)
6 {
7 Iterator<String> i = set.iterator();
8 while (i.hasNext())
9 System.out.print(i.next() + " ");
10 System.out.println();
11

The outputSet
method uses an iterator
to print the contents of
a HashSet<T> object.
Iterators are described
in Section 16.3.

}

12 public static void main(String[] args)
13 {
14 HashSet<String> round = new HashSet<String>();
15 HashSet<String> green = new HashSet<String>();

16 // Add some data to each set
17 round.add("peas");
18 round.add("ball");
19 round.add("pie");
20 round.add("grapes");

21 green.add("peas");
22 green.add("grapes");
23 green.add("garden hose");
24 green.add("grass");