Performance Implications of doing this inside the library vs. doing it in client code:

Performing a transformation on all the keys of the resulting JSON object is much more efficient when done inside the library for two reasons:

1. You can modify the key as you're reading the XML file and generating the JSON object versus generating the entire JSON Object first and then going back to parse the entire structure again and updating the keys.

2. You can provide your own custom transformations which can be handled dynamically thanks to the functional interface built into Java 8.