When using a "mapper", the user tells the library up-front what objects will be used for holding the JSON data. You can do all kinds of optimizations if you kno

When using a "mapper", the user tells the library up-front what objects will be used for holding the JSON data. You can do all kinds of optimizations if you know the shape of the data and what objects will need to be allocated in order to hold it, before the JSON actually arrives.