How to de-dupe a List of Objects?

Question

A Rec object has a member variable called tag which is a String.

If I have a List of Recs, how could I de-dupe the list based on the tag member variable?

I just need to make sure that the List contains only one Rec with each tag value.

Something like the following, but I'm not sure what's the best algorithm to keep track counts, etc:

private List<Rec> deDupe(List<Rec> recs) {

    for(Rec rec : recs) {

         // How to check whether rec.tag exists in another Rec in this List
         // and delete any duplicates from the List before returning it to
         // the calling method?

    }

    return recs;

}

Answer 1

Store it temporarily in a HashMap<String,Rec>.

Create a HashMap<String,Rec>. Loop through all of your Rec objects. For each one, if the tag already exists as a key in the HashMap, then compare the two and decide which one to keep. If not, then put it in.

When you're done, the HashMap.values() method will give you all of your unique Rec objects.

Answer 2

Try this:

private List<Rec> deDupe(List<Rec> recs) {

    Set<String> tags = new HashSet<String>();
    List<Rec> result = new ArrayList<Rec>();

    for(Rec rec : recs) {
        if(!tags.contains(rec.tags) {
            result.add(rec);
            tags.add(rec.tag);
        }
    }

    return result;
}

This checks each Rec against a Set of tags. If the set contains the tag already, it is a duplicate and we skip it. Otherwise we add the Rec to our result and add the tag to the set.