Deleting objects from an ArrayList in Java

Question

I need to delete some objects from an ArrayList if they meet a condition and I\'m wondering which way could be more efficient.

Here\'s the situation: I

Answer 1

There is a hidden cost in removing elements from an ArrayList. Each time you delete an element, you need to move the elements to fill the "hole". On average, this will take N / 2 assignments for a list with N elements.

So removing M elements from an N element ArrayList is O(M * N) on average. An O(N) solution involves creating a new list. For example.

List data = ...;
List newData = new ArrayList(data.size()); 

for (Iterator i = data.iterator(); i.hasNext(); ) {
    Object element = i.next();

    if ((...)) {
        newData.add(element);
    }
}

If N is large, my guess is that this approach will be faster than the remove approach for values of M as small as 3 or 4.

But it is important to create newList large enough to hold all elements in list to avoid copying the backing array when it is expanded.

Answer 2

Another way: The Iterator has an optional remove()-method, that is implemented for ArrayList. You can use it while iterating.

I don't know though, which variant is the most performant, you should measure it.

starblue commented, that the complexity isn't good, and that's true (for removeAll() too), because ArrayList has to copy all elements, if in the middle is an element added or removed. For that cases should a LinkedList work better. But, as we all don't know your real use-cases the best is too measure all variants, to pick the best solution.

Answer 3

You could iterate backwards and remove as you go through the ArrayList. This has the advantage of subsequent elements not needing to shift and is easier to program than moving forwards.

Answer 4

Obviously, of the two methods you mention number 1 is more efficient, since it only needs to go through the list once, while with method number 2 the list has to be traversed two times (first to find the elements to remove, and them to remove them).

Actually, removing a list of elements from another list is likely an algorithm that's worse than O(n) so method 2 is even worse.

The iterator method:

List data = ...;

for (Iterator i = data.iterator(); i.hasNext(); ) {
    Object element = i.next();

    if (!(...)) {
        i.remove();
    }
}

Answer 5

int sizepuede= listaoptionVO.size();
for (int i = 0; i < sizepuede; i++) {
    if(listaoptionVO.get(i).getDescripcionRuc()==null){
        listaoptionVO.remove(listaoptionVO.get(i));
        i--;
        sizepuede--;
     }
}

edit: added indentation

Answer 6

Whilst this is counter intuitive this is the way that i sped up this operation by a huge amount.

Exactly what i was doing:

ArrayList < HashMap < String , String >> results; // This has been filled with a whole bunch of results

ArrayList < HashMap < String , String > > discard = findResultsToDiscard(results);

results.removeall(discard);

However the remove all method was taking upwards of 6 seconds (NOT including the method to get the discard results) to remove approximately 800 results from an array of 2000 (ish).

I tried the iterator method suggested by gustafc and others on this post.

This did speed up the operation slightly (down to about 4 seconds) however this was still not good enough. So i tried something risky...

 ArrayList < HashMap < String, String>> results;

  List < Integer > noIndex = getTheDiscardedIndexs(results);

for (int j = noIndex.size()-1; j >= 0; j-- ){
    results.remove(noIndex.get(j).intValue());
}

whilst the getTheDiscardedIndexs save an array of index's rather then an array of HashMaps. This it turns out sped up removing objects much quicker ( about 0.1 of a second now) and will be more memory efficient as we dont need to create a large array of results to remove.

Hope this helps someone.