Re: Copying collection without duplicates

Patricia Shanahan <>
Sat, 11 Aug 2007 12:01:10 -0700
Karsten Wutzke wrote:

On 11 Aug., 00:29, Danno <> wrote:

On Aug 10, 8:30 am, Karsten Wutzke <> wrote:

I have the following method overriding Collection.addAll:
public boolean addAll(int index, Collection<? extends E> cln)
        if ( containsAll(cln) )
                return false;
        //build list without dupes (always)
        ArrayList<E> al = new ArrayList<E>(cln.size());
        Iterator<? extends E> itr = cln.iterator();
        while ( itr.hasNext() )
                E elem =;
                if ( !contains(elem) )
        cln = al;
        //allows dupes and nulls
        return super.addAll(index, cln);
Is there any faster way without overriding other methods?


Set<?> uniqueCollection = new TreeSet(collection);

Hmm how does this skip duplicates?

A set contains no duplicates, so if the collection were the list "A",
"B", "A" the treeset would contain "A", "B". However, the TreeSet
iterator is in compareTo order, not the List order.

If the collection is too large for linear scanning, I would implement
the no-duplicates list using two data structures, a HashSet for
determining which elements are eligible for adding, and a List to
preserve order.


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