How much memory does a hashtable use?

Question

Would a hashtable/hashmap use a lot of memory if it only consists of object references and int's?

As for a school project we had to map a database to objects (that's what being done by orm/hibernate nowadays) but eager to find a good way not to store id's in objects in order to save them again we thought of putting all objects we created in a hashmap/hashtable, so we could easily retrieve it's ID. My question is if it would cost me performance using this, in my opinion more elegant way to solve this problem.

Answer 1

Would a hashtable/hashmap use a lot of memory if it only consists of object references and int's?

"a lot" depends on how many objects you have. For a few hundreds or a few thousands, you're not going to notice.

But typically the default Java collections are really incredibly inefficient when you're working with primitives (because of the constant boxing/unboxing from "primitive to wrapper" going on, like say "int to Integer") , both from a performances and memory standpoint (the two being related but not identical).

If you have a lot of entries, like hundreds of thousands or millions, I suggest using for example the Trove collections.

In your case, you'd use this:

TIntObjectHashMap<SomeJavaClass>

or this:

TObjectIntHashMap<SomeJavaClass>

In any case, that shall run around circle the default Java collections perf-wise and cpu-wise (and it shall trigger way less GC, etc.).

You're dodging the unnecessary automatic (un)boxing from/to int/Integer, the collections are creating way less garbage, resizing in a much smarter way, etc.

Don't even get me started on the default Java HashMap<Integer,Integer> compared to Trove's TIntIntHashMap or I'll go berzerk ;)