code: public String getSkeleton(String pattern) {
          synchronized (this) { // synchronized since a getter must be thread-safe
              current.set(pattern, fp, false);
              return current.toString();
          }
      }
docs: Utility to return a unique skeleton from a given pattern. For example,
  both "MMM-dd" and "dd/MMM" produce the skeleton "MMMdd".
  
  @param pattern Input pattern, such as "dd/MMM"
  @return skeleton, such as "MMMdd"