Not able searialize HashMap and adding instance into JSON about google-gson HOT 7 CLOSED

I meant HashMap key where key is userdefined not able to serialize properly.

Definitely a problem, but the solution isn't obvious.

Perhaps GSON should look at the conversion for the map's key type. If it 
converts to a primitive, then a GSON could emit an object. If it converts to 
another type, GSON could emit an array of pairs. For example, the customer 
above could be converted to this:
  [
    [{ "customerId": 1, "name": "John" },{"campaignId": 1532, "name": "25-2"}]
  ]
We could emit something more concise for say, a map with integer keys:
  { "5": "five", "6": "six" }

Thanks for reply! Is it going to be fixed in GSON library soon?

 [
    [{ "customerId": 1, "name": "John" },{"campaignId": 1532, "name": "25-2"}]
  ]

I'm not able to understand how to convert?

I've Hashmap<Customer, List<Campaign>>, in this case how do I put above one as 
a key?

I'm not sure about when it'll be fixed. In the interim you can work around the 
problem by writing a custom JsonSerializer.

This is basically a dupe of issue 210, which provides an example patch that 
solves this problem.