How to test if JSON object is empty in Java

obj.length() == 0

is what I would do.

If you're okay with a hack -

obj.toString().equals("{}");

Serializing the object is expensive and moreso for large objects, but it's good to understand that JSON is transparent as a string, and therefore looking at the string representation is something you can always do to solve a problem.

If empty array:

.size() == 0

if empty object:

.length() == 0