Listing 1: Person Class

1  // Person Class
2  public class Person {
3    private String ssn;
4
5    public Person(String newSSN) {
6      this.ssn = newSSN;
7    }
8
9    public String toString() {
10     return ssn;
11   }
12 }


Listing 2: PedanticPerson

1  // PedanticPerson Class overrides
2  // hashCode and equals.
3  public class PedanticPerson 
4      extends Person {
5
6    public PedanticPerson(
7      String newSSN) {
8      super(newSSN);
9    }

10   public int hashCode() {
11     return getSSN().hashCode();
12   }

13   public boolean equals(
14     Object obj) {
15     Person p = (Person) obj;
16
17     return getSSN().equals(
18        p.getSSN());
19   }
20 }


Listing 3: Main Program

1  import java.util.HashSet;
2  import java.util.Set;
3
4  public class HashSetExample {
5
6    public static void main(
7      String[] args) {
8
9      Set dupSet = new HashSet();
10     dupSet.add(new 
11          Person("000-11-1111"));
12       dupSet.add(new
13          Person("222-23-1234"));
14       dupSet.add(new
15          Person("000-11-1111"));
16
17     // has duplicates
18     System.out.println(dupSet);

19     Set set = new HashSet();
20     set.add(new 
21     PedanticPerson("000-11-1111"));
22     set.add(new
23     PedanticPerson("222-23-1234"));
24     set.add(new
25     PedanticPerson("000-11-1111"));
26
27     // has no duplicates
28     System.out.println(set);
29
30   }
31 }

Listing 4: PersonComparator

1  // PersonComparator will be used
2  // to sort the Persons in
3  // descending order based
4  // on SSN.
5  public class PersonComparator
6    implements Comparator {
7
8    public int compare(
9      Object o1,
10     Object o2) {
11
12     Person p1 = (Person) o1;
13     Person p2 = (Person) o2;
14
15     String ssn1 = p1.getSSN();
16     String ssn2 = p2.getSSN();
17
18     return ssn2.compareTo(ssn1);
19   }
20 }