Re: remove duplicates?

Eric Sosman <esosman@ieee-dot-org.invalid>
Mon, 05 Sep 2011 08:38:19 -0400
On 9/5/2011 4:44 AM, bob wrote:

Let's say you have a Vector of String objects. What is the easiest
way to remove duplicates?

     The easiest way is to call the Vector's clear() method, which will
remove all duplicates. (It will also remove everything else, but if
the criterion is "easiest" this is surely the winner.)

     If by "remove duplicates" you mean "retain one and only one
instance of each unique String," you can use a Set:

    Vector<String> oldVec = ...;
    Vector<String> newVec = new Vector<String>(
        new HashSet<String>(oldvec));

Two things to note: First, this approach will do as advertised, but
will also scramble whatever order there may have been in oldVec.
Second, if there are five "X"'s in oldVec, there's no guarantee which
of them will get into newVec -- it could be any of the five.

     If by "remove duplicates" you mean "retain only those Strings
that are unique, discarding all pairs, triples, et cetera," I know
of no pre-canned solution. You could sort the Vector and then sweep
over it looking for adjacent identical Strings. Or you could use a
pair of Sets and two passes, something like

    Vector<String> vec = ...;
    Set<String> seen = new HashSet<String>();
    Set<String> dups = new HashSet<String>();
    for (String s : vec) {
        if (!seen.add(s)) {
            dups.add(s); // second or subsequent sighting
    for (Iterator<String> it = vec.iterator(); it.hasNext(); ) {
        String s =;
        if (dups.contains(s)) {

     Incidentally, Vector fell out of fashion several years ago.
Nowadays, the cognoscenti use List and its implementations.

Eric Sosman

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