DecimalFormat says that:

   NaN is formatted as a single character, typically \uFFFD. This
   character is determined by the DecimalFormatSymbols object. This is
   the only value for which the prefixes and suffixes are not used.

But DecimalFormatSymbols.getNaN() returns a String. It should be changed to be able to accommodate "NaN".

See also http://www.unicode.org/review/#pri74 .

Infinity has the same problem.