HashMap intialization parameters (load / initialcapacity)

Question

What values should I pass to create an efficient HashMap / HashMap based structures for N items?

In an ArrayList, the efficient number is N (N already assumes future grow). What should be the parameters for a HashMap? ((int)(N * 0.75d), 0.75d)? more? less? What is the effect of changing the load factor?

Answer 1

Regarding the load factor, I'll simply quote from the HashMap javadoc:

As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

Meaning, the load factor should not be changed from .75 , unless you have some specific optimization you are going to do. Initial capacity is the only thing you want to change, and set it according to your N value - meaning (N / 0.75) + 1, or something in that area. This will ensure that the table will always be large enough and no rehashing will occur.

Answer 2

In an ArrayList, the efficient number is N (N already assumes future grow).

Erm, no it doesn't, unless I misunderstand what you're saying here. When you pass an integer into the Arraylist constructor, it will create an underlying array of exactly that size. If it turns out you need even a single extra element, the ArrayList will need to resize the underlying array when you next call add(), causing this call to take a lot longer than it usually would.

If on the other hand you're talking about your value of N taking into account growth - then yes, if you can guarantee the value will never go above this then calling such an Arraylist constructor is appropriate. And in this case, as pointed out by Hank, the analogous constructor for a map would be N and 1.0f. This should perform reasonably even if you do happen to exceed N (though if you expect this to occur on a regular basis, you may wish to pass in a larger number for the initial size).

The load factor, in case you weren't aware, is the point at which the map will have its capacity increased, as a fraction of the total capacity.

Edit: Yuval is probably right that it's a better idea to leave the load factor around 0.75 for a general purpose map. A load factor of 1.0 would perform brilliantly if your keys had sequential hashcodes (such as sequential integer keys), but for anything else you will likely run into collisions with the hash buckets, meaning that lookups take longer for some elements. Creating more buckets than is strictly necessary will reduce this chance of collision, meaning there's more chance of elements being in their own buckets and thus being retrievable in the shortest amount of time. As the docs say, this is a time vs space tradeoff. If either is particularly important to you (as shown by a profiler rather than prematurely optimising!) you can emphasize that; otherwise, stick with the default.

Answer 3

It's also notable that having a HashMap on the small side makes hash collisions more likely, which can slow down lookup. Hence, if you really worry about the speed of the map, and less about its size, it might be worth making it a bit too large for the data it needs to hold. Since memory is cheap, I typically initialise HashMaps for a known number of items with

HashMap<Foo> myMap = new HashMap<Foo>(numberOfElements * 2);

Feel free to disagree, in fact I'd quite like to have this idea verified or thrown out.

Answer 4

I asked a similar question relating to .NET generic Dictionary recently. You might find the discussion interesting there too.

Answer 5

Referring to HashMap source code will help.

If the number of entries reaches threshold(capacity * load factor), rehashing is done automatically. That means too small load factor can incur frequent rehashing as entries grow.

Answer 6

For very large HashMaps in critical systems, where getting the initial capacity wrong can be very problematic, you may need empirical information to determine how best to initialize your Map.

CollectionSpy (collectionspy.com) is a new Java profiler which lets you see in the blink of an eye which HashMaps are close to needing rehashing, how many times they have been rehashed in the past, and more. An ideal tool to determine safe initial capacity arguments to capacity-based container constructors.

Answer 7

The answer Yuval gave is only correct for Hashtable. HashMap uses power-of-two buckets, so for HashMap, Zarkonnen is actually correct. You can verify this from the source code:

  // Find a power of 2 >= initialCapacity
  int capacity = 1;
  while (capacity < initialCapacity)
  capacity <<= 1;

So, although the load factor of 0.75f is still the same between Hashtable and HashMap, you should use an initial capacity n*2 where n is the number of elements you plan on storing in the HashMap. This will ensure the fastest get/put speeds.

Answer 8

It's safe in most cases of List and Map initialization to make the List or Map with the following size params.

List<T>(numElements + (numElements / 2));
Map<T,T>(numElements + (numElements / 2));

this follows the .75 rule as well as saves a little overhead over the * 2 operation described above.