[merb]

well only because java has no value types/struct like data structure.

the reason why this makes memory so heavy is because even a simple int list, needs boxing. So you end up with a big chunk of garbage which you don't need. On the server, this is not a problem because most of the time the list is a young gen object and die really fast, so G1GC solves most problems. The problem of course is really big, because arrays are a pain to work with and not many people do that.

However as soon as java 10 hits, we might get value types and maybe this changes the game (but we will see, since it would've taken way to long for that thing). Also there are more and more types inside the standard library who would be great value types, i.e. LocalDate/LocalDateTime, but at the moment they take way more memory than needed, especially since they only contain 2 immutable shorts and 1 immutable int.. in the perfect case it would be something like 8 byte, however it takes way way way way more. (basically a simple class at least takes 16 bytes, which is already double the field size, so you end up with 24 bytes [probably more due to various other stuff])

btw. for a game like minecraft you probably have a lot of types that follow the same stuff like LocalDate, small class with immutable int's/short's for position, etc. these have the same problem and will fill the memory more quickly than needed