code: public void setCharset(Charset charset)
      {
          mutationCheck();
          if (charset == null
              || charset.equals(ASCII)
              || charset.equals(UTF8))
          {
              myCharset = charset;
          }
          else
          {
              throw new IllegalArgumentException("Unsupported Charset "
                                                 + charset);
          }
      }
docs: Sets the charset denoting the output encoding.
  Only ASCII and UTF-8 are supported; applications can use the helper
  constants {@link #ASCII} and {@link #UTF8}.
  
  @param charset may be null, denoting the default of UTF-8.
  
  @see #getCharset()
  @see #withCharset(Charset)
  
  @throws UnsupportedOperationException if this is immutable.