How to efficiently (performance) remove many items from List in Java?

Question

I have quite large List named items (>= 1,000,000 items) and some condition denoted by that selects items to be deleted and is true for many (maybe hal

Answer 1

I would imagine that building a new list, rather than modifying the existing list, would be more performant - especially when the number of items is as large as you indicate. This assumes, your list is an ArrayList, not a LinkedList. For a non-circular LinkedList, insertion is O(n), but removal at an existing iterator position is O(1); in which case your naive algorithm should be sufficiently performant.

Unless the list is a LinkedList, the cost of shifting the list each time you call remove() is likely one of the most expensive parts of the implementation. For array lists, I would consider using:

public static  List removeMany(List items) {
    List newList = new ArrayList(items.size());
    Iterator iter = items.iterator();
    while (iter.hasNext()) {
        T item = iter.next();
        //  goes here
        if (/*: */i++ % 2 != 0) {
            newList.add(item);
        }
    }
    return newList;
}