How can I find out the size of a java.util.List if the actual size is greater than the Integer.MAX_VALUE?

Question

As I understood java.util.List does not have size restrictions. As I understand, the size() method:

Returns the number of elements in this list. If this list contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Therefore I have following question:

If my List has a size that is larger than the Integer.MAX_VALUE, is there a way to determine the exact size?

Answer 1

You would need to count them yourself, using a bigger data type.

long realSize;

if (myList.size() < Integer.MAX_VALUE) {
    realSize = myList.size();
} else {
    realSize = Integer.MAX_VALUE;
    ListIterator<Foo> iter = myList.listIterator(Integer.MAX_VALUE);
    while (iter.hasNext()) {
      realSize++;
      iter.next();
    }
 }

Answer 2

It would depend on the specific implementation of List you are using. For example ArrayList uses a java array internally, which has a max size of Integer.MAX_VALUE - 5. Other implementations of List might have different max sizes.

Answer 3

If a list supports sizes that huge, it will almost certainly support some sort of alternate size method you can use to get the size as a long. For example, the first such implementation I found, HugeCollections, has a longSize method. The overhead of trying to get the true size without such a method is simply absurd.

I don't think any standard library List implementation supports sizes that huge; ArrayList and LinkedList don't. If you ever need to deal with this kind of problem, the documentation for the giant list type you're using should tell you what to do.