Java HashMap vs JSONObject

As you said, JSONObject is backed by a HashMap.

Because of this, performance will be almost identical. JSONObject.get() adds a null check, and will throw an exception if a key isn't found. JSONObject.put() just calls map.put().

So, there is almost no overhead. If you are dealing with JSON objects, you should always use JSONObject over HashMap.

I would say the question doesn't make sense for a few reasons:

1. Comparing apples to oranges: HashMap and JSONObject are intended for 2 completely different purposes. It's like asking "is the Person class or Company class more efficient for storing a PhoneNumber object". Use what makes sense.
2. If you are converting to/from JSON, you are likely sending the data to a far away place (like a user's browser). The time taken to send this data over the network and evaluate it in the user's browser will (likely) far eclipse any performance differences of populating a Hashmap or JSONObject.
3. There is more than 1 "JSONObject" implementation floating around out there.
4. Finally, you haven't asked about what sort of performance you would like to measure. What are you actually planning to do with these classes?