code: @GwtCompatible(serializable = true)
    public static <T> Ordering<T> from(Comparator<T> comparator) {
      return (comparator instanceof Ordering)
          ? (Ordering<T>) comparator
          : new ComparatorOrdering<T>(comparator);
    }
docs: Returns an ordering based on an <i>existing</i> comparator instance. Note that it is
  unnecessary to create a <i>new</i> anonymous inner class implementing {@code Comparator} just
  to pass it in here. Instead, simply subclass {@code Ordering} and implement its {@code compare}
  method directly.
  
  <p><b>Java 8 users:</b> this class is now obsolete as explained in the class documentation, so
  there is no need to use this method.
  
  @param comparator the comparator that defines the order
  @return comparator itself if it is already an {@code Ordering}; otherwise an ordering that
  wraps that comparator