map := Int:Int[:]
map[0] = 0    
echo(util::JsonOutStream.writeJsonToStr(map))

Result:

sys::CastErr: java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.String
  fan.sys.Map.each (Map.java:347)
  util::JsonOutStream.writeJsonMap (JsonOutStream.fan:86)
  util::JsonOutStream.writeJson (JsonOutStream.fan:50)
  util::JsonOutStream.writeJsonToStr (JsonOutStream.fan:24)

It isn't really a bug. JSON only supports string keys for its maps, and the input here is Int keys. We could potentially convert any key type to strings, but that doesn't seem like the correct behavior (better to do that outside of JSON API yourself).

Could this ClassCastException not be caught and a more informative exception thrown instead? Something with a message like, "JSON only supports Str keys for maps" .

I've always been a keen believer of delivering as much context info to the developer as possible - esp when writing frameworks.

+1 for more informative exception

+1 for more informative exception

agreed, I pushed a fix