Question regarding javacertificate.com study question

[David Willis]

I have some concerns regarding a question I saw at javacertificate.com. Here's the question.

What will be the output of the following code if you try to execute it? Knowing that a set cannot contain duplicate entries.
1.  public class Computer implements Comparable{
2.     int totalCpus = 0;
3.     public int compareTo(Object o1){
4.        throw new UnsupportedOperationException();
5.     }
6.     public Computer(int totalCpus){
7.        this.totalCpus=totalCpus;
8.     }
9.     public int hashCode( ){
10.       return new Integer(totalCpus).hashCode( );
11.    }
12.    public static void main(String[] args){
13.       Set lhs = new HashSet();
14.       lhs.add(new Computer(1));
15.       lhs.add(new Computer(2));
16.       lhs.add(new Computer(1));
17.       System.out.println (lhs.size());
18.    }
19. }
1.  Compilation succeeds, the output is 2
2.  Compilation succeeds, the output is 3
3.  Compile time error, the class should define the equals() method
4.  Compile time error, the class should define the compare() method
5.  Compilation succeeds, UnsupportedOperationException is thrown 

I believe the answer is (4), since the class Computer is supposed to implement the Comparable interface, but neither implements " int compare(Object, Object) " nor is declared abstract. But according to javacertificate.com, the answer is (2). What am I missing, or is javacertificate.com wrong? Thanks.

[Roger Chung-Wee]

The Comaparable interface has one method:
public int compareTo(Object o)
This is indeed implemented. I do not believe, however, that it is ever invoked as it seems to be used by sort methods of the Collections and Array classes.
Three objects are added to the HashSet on the basis of them not being the same (not having the same object references) according to the equals method inherited from Object.

[David Willis]

OK, I see what I did. I got the Comparable and Comparator interfaces mixed up.
Thanks.