code: @Override
      public StringBuffer format(Object number, StringBuffer toAppendTo,
              FieldPosition pos) {
          if (!(number instanceof Number)) {
              throw new IllegalArgumentException("'" + number + "' is not a Number");
          }
          Number numberObject = (Number) number;
          toAppendTo.append(format(numberObject, numberObject.doubleValue()));
          return toAppendTo;
      }
docs: Formats a plural message for a given number and appends the formatted
  message to the given <code>StringBuffer</code>.
  @param number a number object (instance of <code>Number</code> for which
  the plural message should be formatted. If no pattern has been
  applied to this <code>PluralFormat</code> object yet, the
  formatted number will be returned.
  Note: If this object is not an instance of <code>Number</code>,
  the <code>toAppendTo</code> will not be modified.
  @param toAppendTo the formatted message will be appended to this
  <code>StringBuffer</code>.
  @param pos will be ignored by this method.
  @return the string buffer passed in as toAppendTo, with formatted text
  appended.
  @throws IllegalArgumentException if number is not an instance of Number