code: public static synchronized CoderResult unmappableForLength(int length)
              throws IllegalArgumentException {
          if (length > 0) {
              Integer key = Integer.valueOf(length);
              synchronized (_unmappableErrors) {
                  CoderResult r = _unmappableErrors.get(key);
                  if (r == null) {
                      r = new CoderResult(TYPE_UNMAPPABLE_CHAR, length);
                      _unmappableErrors.put(key, r);
                  }
                  return r;
              }
          }
          throw new IllegalArgumentException("length <= 0: " + length);
      }
docs: Gets a <code>CoderResult</code> object indicating an unmappable
  character error.
  
  @param length
  the length of the input unit sequence denoting the unmappable
  character.
  @return a <code>CoderResult</code> object indicating an unmappable
  character error.
  @throws IllegalArgumentException
  if <code>length</code> is non-positive.