code: public int nextIndex() {
  		int index = -1;
  		for (int i = 0; i <  _values.length && index == -1; ++i) {
  			if (Double.isNaN(_values[i])) {
  				index = i;
  			}
  		}
  
  		return index;
  	}
docs: Return the index of the next free {@code double} <i>slot</i>. If all
  <i>slots</i> are occupied, {@code -1} is returned.
  
  @return the index of the next free {@code double} <i>slot</i>, or
  {@code -1} if all <i>slots</i> are occupied