I recently came into a situation when sending a nested object from Java to Flex via a HashMap the Objects were null. More precisely:
final Map<Integer, List<String>> tempMap = new HashMap<Integer, List<String>>();
would send the keys as integers but the values were all null.
But when sending the same with String keys:
final Map<String, List<String>> tempMap = new HashMap<String, List<String>>();
the objects came thru.
Are there any restrictions in BlazeDS serialization when using complex types as keys?
Just in case anyone is still looking for an answer to this issue, I just ran into the same problem. During my investigation, I learned that BlazeDS serialization is expecting only String key values for Map objects. Since the Map coming in has Integer keys, the key is changed to a string by the serialization. At that point, the value cannot be obtained and serialized. Change the Integer key to a String and it works. Here is the site that helped me get it working.