How to retain/get only the duplicate elements in a list ~ foundjava

Let’s say you have a list of duplicate (and non-duplicate) items, and you want a new collection with only the duplicate items in there. The easiest way is to extend the HashSet.

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public class DuplicatesOnlySet<E> extends HashSet<E> {     private final Set<E> uniques = new HashSet<E>();       public  DuplicatesOnlySet(Collection<? extends E> c)     {         super.addAll(c);     }       @Override     public boolean add(E e)     {                        if(!this.uniques.add(e))              return super.add(e);       return false;     } }

Call it like

1	List<String> duplicates = new ArrayList<String>(new DuplicatesOnlySet<String>(original)) ;

where original is the Collection with the duplicate items.