How to sort HashSet containing custom Objects...?

[Paresh Wankhede]

Dear All,
I am in a situation where I need to sort my HashSet. The HashSet contains our custom class objects. Let me explain with the help of example,.

e.g.
I have Employee Class with age and fullName as its instance variable. I am adding the instances of Employee class into my HashSet. Now at one point of time, I need to sort out the the HashSet on the basis of the SurName component of the Employee object. And the end result should be: when i iterate over the HashSet, I should be able to get the Employee object unchanged with their full name, but they are sorted in natural order on the basis of surname.


Please Help

.. paresh

[Campbell Ritchie]

Sets don't have an order. Can you sort a Set at all? That sorting sounds like what you do with a List.

[Rob Spoor]

Campbell Ritchie wrote:Sets don't have an order. Can you sort a Set at all?

How about TreeSet?
But you are right about HashSet - that has no defined order, and LinkedHashSet only has insertion (and optionally access) order, nothing custom.

Paresh, you can use a List like Campbell suggested, then sort it using Collections.sort and a small set of custom java.util.Comparator implementations. Or you can create a temporary TreeSet and use its iterator. For instance:

public static <E> Iterator<E> iterator(Set<E> set, Comparator<? super E> comparator)
{
    TreeSet<E> treeSet = new TreeSet<E>(comparator);
    treeSet.addAll(set);
    return treeSet.iterator();
}

[Campbell Ritchie]

Yes, I forgot about TreeSet, Rob.

[Paresh Wankhede]

Hmmmm....
But is there no way of sorting the HashSet as we have in TreeSet?

I have thought of using TreeSet but trying to sort it out using HashSet only.
Please advise...

.. paresh

[Rob Spoor]

As said before, HashSet has an undefined ordering, based on the hash codes but even these are used in a modified form by the HashSet. As such, the ordering is going to be unspecified and there is nothing you can do about that. If you need ordering other than insertion order then TreeSet is the only solution provided by the core API.

[Campbell Ritchie]

You can always create a TreeSet and a Comparator and copy all the entries across. You may be able to pass one collection to the constructor of the second.