ArrayList.ListIterator(int index) vs ArrayList.get(int index)

I was wondering what the performance impact would be when using ArrayList.ListI开发者_JS百科terator(int index - 1), then it.next() in contrast to using ArrayList.get(int index)?

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Why look at the implementations...

1: List.listIterator(int)

public ListIterator<E> listIterator(final int index) {
if (index<0 || index>size())
  throw new IndexOutOfBoundsException("Index: "+index);

return new ListItr(index);
}

with

private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
    cursor = index;
}

// [...]

and

public E next() {
        checkForComodification();
    try {
    E next = get(cursor);
    lastRet = cursor++;
    return next;
    } catch (IndexOutOfBoundsException e) {
    checkForComodification();
    throw new NoSuchElementException();
    }
}

2: List.get(int)

public E get(int index) {
RangeCheck(index);

return (E) elementData[index];
}

Should be quite obvious, which one is faster. For details about performance impacts, I will have to agree with Mike. Profile it. Whatever the reason is, you'd like to use such a distinct access method just to access one item (?)

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Profiled it on my machine, with an ArrayList of 10,000 integers.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Random;

/**
 *
 * @author Michael Drogalis
 */
public class Launcher {

    public static void main(final String[] args) {
        List<Integer> sut = new ArrayList<Integer>();

        Random generator = new Random();
        for (int k = 0; k < 100000; k++) {
            sut.add(generator.nextInt(64));
        }

        testGetMethod(sut, generator);
        testIteratorMethod(sut, generator);
    }

    private static void testGetMethod(List<Integer> aList, Random aGenerator) {
        for (int k = 0; k < 100000; k++) {
            aList.get(aGenerator.nextInt(1000));
        }
    }

    private static void testIteratorMethod(List<Integer> aList, Random aGenerator) {
        for (int k = 0; k < 100000; k++) {
            Iterator iterator = aList.listIterator(Math.abs(aGenerator.nextInt(100000) - 1));
            iterator.next();
        }
    }
}

The get method took 6.47 ms to complete 10,000 fetches. The Iterator style took 18.7 ms to completely 10,000 fetches.

They differ by a factor of nearly 3.

Edit: Profiled each method 1,000 times. Here are some particularly interesting results: get takes 2403 ms. Iterator takes 3,661 ms.

Now it's a factor of 1.5. Interesting...

Reasonable results - for my machine. Test at your own risk.

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Both find the result in O(1) (iaw - time does not depend on size of list). So if you just use it once, you won't notice a difference on small or big collections.

But the "ListIterator" solution creates extra objects and that is a waste of memory, because we have the get method that just takes the requested object from the backing array.