We really should remove that map because now they can be used multi threaded because for example the DoubleConverter has a static INSTANCE variable..
The AbstractIntegerConverter doesnt do that caching for example, that returns a new Format instance everytime.
The problem is that we can remove that map just fine, expects that we have setNumberFormat(Locale,Format) method
thats is a method that is very wrong because ot the caching, And they should use/override getNumberFormat when you call setNumberFormat..