code: private static double[] collectDomain(NonBlockingHashMapLong ls) {
      int sz = ls.size();         // Uniques
      double ds[] = new double[sz];
      int x = 0;
      for (NonBlockingHashMapLong.IteratorLong i = iter(ls); i.hasNext(); )
        ds[x++] = Double.longBitsToDouble(i.nextLong());
      Arrays.sort(ds);
      return ds;
    }
docs: them as a sorted double[].