Mapping one JSON message format to another in Java
It's always a bad sign if you do have different levels of presentations inside a class. 

It would be much better to separate the concerns and implement based on the "single responsibility principle".

That would mean that you have the class DeliveredCommunication which holds a set of Strings/Objects for the messages. This is also known as a DTO (Data Transfer Object). 

Everyone who wants to use it, can now decide how the presentation should look like. 

Transforming data to a specific exchange format should be done at the level of your application where the exchange happens. 

Hope it is clear enough and that it helps 

P.S.: I would change DeliveredCommunicaton to Communication with a property called state which could be realized with an enum of which one value would be 'delivered'.