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non duplicates in a Set

Rob McBryde

Joined: Dec 18, 2010
Posts: 16
Hi All,

I am in the process of writing a small program where I have created a very simple class called Vertex that has a String as a field. I am now attempting to add a number of instances of my Vertex class into a HashSet. However, my resultant HashSet has duplicates in it.

I have added a equals method and compareTo method in my Vertex class to attempt to allow my HashSet to uniquely identify Vertexes but I am clearly missing something. Can anyone point me in the right direction. Do I need to override the HashCode in my Vertex class?

This results in a Set of [A, B, A, C] when I would like to have no duplicates, hence my choice in using a Set.

Thanks in advance
Matthew Brown

Joined: Apr 06, 2010
Posts: 4543

Rob McBryde wrote:Do I need to override the HashCode in my Vertex class?

Yes, that's it. You should always override hashCode() whenever you override equals(), as they should be consistent with each other (see the contract descriptions in the Javadocs for Object). HashSet relies on hashCode() working correctly (the "Hash" in the name is not a coincidence!).
Rob Spoor

Joined: Oct 27, 2005
Posts: 20271

And your equals method has a huge bug. If you pass null or an instance of an incompatible class the method should return false. Your implementation will throw a NullPointerException / ClassCastException.

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Stephan van Hulst

Joined: Sep 20, 2010
Posts: 4597

Not only is it bugged, it uses a poor construct:

The mind is a strange and wonderful thing. I'm not sure that it will ever be able to figure itself out, everything else, maybe. From the atom to the universe, everything, except itself.
Rob McBryde

Joined: Dec 18, 2010
Posts: 16
Thanks for the advice, I have re-written my equals method and overridden the hashCode() method within my Vector class. Have tested this and no duplicate Vectors are being added to my HashSet. Hurray!

I appreciate the comments, a little unsure about Stephan's comment about the poor construct of my initial equals method. Would you be able to further advise why it is poor and what I should be doing to improve it ?

Paul Beckett
Ranch Hand

Joined: Jun 14, 2008
Posts: 96
I think what Stephan is getting at is that you can replace:


thus improving the readability of your code.
Rob McBryde

Joined: Dec 18, 2010
Posts: 16
Thanks Paul,

Makes perfect sense to me now. I've updated my code and will I ensure I stick to the more readable and less verbose approach from now on.

I agree. Here's the link:
subject: non duplicates in a Set
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