This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
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.
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!).
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.
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.
Joined: Dec 18, 2010
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 ?