Which way of removing elements from an ArrayList is more efficient?

Question

I have the following declarations:

ArrayList<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
list.add("2");
list.add("3");
list.add("4");

Now my question is: if I want to remove the "2"s from the list, which way is better?

first way:

for(int i = 0; i < list.size(); i++) {
    if(list.get(i).equals("2")) {
        list.remove(i);
        i--;
    }
}

second way:

Iterator<String> iterator = list.iterator();
    while(iterator.hasNext())
        if(iterator.next().equals("2"))
            iterator.remove();

Are both safed properly and which is more efficient?

Are there any other methods to remove elements from an ArrayList without getting IndexOutOfBounds errors?

Answer 1

Actually, what might be faster is

list.removeAll(Collections.singleton("2"));

Behind the scenes, for an ArrayList, it does basically create a new copy of the array like @Edmund suggests, but at a lower level, which may lead to higher performance.

Still, as others have mentioned, a LinkedList generally has better performance for removing multiple elements from a large list.

(Even if you do decide to switch to a LinkedList you can still use the above code, it will be equivalent to using the iterator method with a little bit of overhead for the singleton collection that's created and for some extra method calls that happen.)