Remove objects from a java set

How do i remove all the objects from one set, that exist also in an other set. for exaple:

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SET 1 = { a,b,c,d,e,f)  |
                        |---> NEW SET 1= (a,b,d,e}
SET 2 = {c,f)           |

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I'm not sure what the problem is

  set1.removeAll(set2);

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code from org.apache.commons.collections.CollectionUtils

  public static Collection subtract(final Collection a, final Collection b) {
        ArrayList list = new ArrayList( a );
        for (Iterator it = b.iterator(); it.hasNext();) {
            list.remove(it.next());
        }
        return list;
    }