Java中的内存高效稀疏数组

Question

(There are some questions about time-efficient sparse arrays but I am looking for memory efficiency.)

I need the equivalent of a List<T> or Map<Integer,T> which

1. Can grow on demand just by setting a key larger than any encountered before. (Can assume keys are nonnegative.)
2. Is about as memory-efficient as an ArrayList<T> in the case that most of the indices are not null , ie when the actual data is not very sparse.
3. When the indices are sparse, consumes space proportional to the number of non- null indices.
4. Uses less memory than HashMap<Integer,T> (as this autoboxes the keys and probably does not take advantage of the scalar key type).
5. Can get or set an element in amortized log(N) time where N is the number of entries: need not be linear time, binary search would be acceptable.
6. Implemented in a nonviral open-source pure Java library (preferably in Maven Central).

Does anyone know of such a utility class?

I would have expected Commons Collections to have one but it did not seem to.

I came across org.apache.commons.math.util.OpenIntToFieldHashMap which looks almost right except the value type is a FieldElement which seems gratuitous; I just want T extends Object . It looks like it would be easy to edit its source code to be more generic, though I would rather use a binary dependency if one is available.

Answer 1

我会尝试用宝库收藏，有TIntObjectMap它可以为您的意图工作。

Answer 2

I would look at Android's SparseArray implementation for inspiration. You can view the source by downloading AOSP's source code here http://source.android.com/source/downloading.html

Answer 3

I have saved my test case as jglick/inthashmap . The results:

HashMap size: 1017504
TIntObjectMap size: 853216
IntHashMap size: 846984
OpenIntObjectHashMap size: 760472

Answer 4

I will suggest you to use OpenIntObjectHashMap from Colt library. Link

Answer 5

Late to this question, but there is IntMap in libgdx which uses cuckoo hashing . If anything it would be interesting to compare with the others.