Memory overhead of Java HashMap compared to ArrayList

Question

I am wondering what is the memory overhead of java HashMap compared to ArrayList?

Update:

I would like to improve the speed for searching fo

Answer 1

Hashmaps try to maintain a load factor (usually 75% full), you can think of a hashmap as a sparsely filled array list. The problem in a straight up comparison in size is this load factor of the map grows to meet the size of the data. ArrayList on the other hand grows to meet it's need by doubling it's internal array size. For relatively small sizes they are comparable, however as you pack more and more data into the map it requires a lot of empty references in order to maintain the hash performance.

In either case I recommend priming the expected size of the data before you start adding. This will give the implementations a better initial setting and will likely consume less over all in both cases.

Update:

based on your updated problem check out Glazed lists. This is a neat little tool written by some of the Google people for doing operations similar to the one you describe. It's also very quick. Allows clustering, filtering, searching, etc.