Question

I have an ArrayList, and I need to filter it (only to remove some elements).

I can’t modify the original list.

What is my best option regarding performances :

• Recreate another list from the original one, and remove items from it :

code :

List<Foo> newList = new ArrayList<Foo>(initialList);
for (Foo item : initialList) {
    if (...) {
        newList.remove(item);
    }
}

• Create an empty list, and add items :

code :

List<Foo> newList = new ArrayList<Foo>(initialList.size());
for (Foo item : initialList) {
    if (...) {
        newList.add(item);
    }
}

Which of these options is the best ? Should I use anything else than ArrayList ? (I can’t change the type of the original list though)

As a side note, approximatively 80% of the items will be kept in the list. The list contains from 1 to around 20 elements.

Answer 1

Since I’m only get suggestions here, I decided to run my own bench to be sure.

Here are the conclusions (with an ArrayList of String).

• Solution 1, remove items from the copy : 2400 ms.

• Solution 2, create an empty list and fill it : 1600 ms. newList = new ArrayList<Foo>();

• Solution 3, same as 2, except you set the initial size of the List : 1530 ms. newList = new ArrayList<Foo>(initialList.size());

• Solution 4, same as 2, except you set the initial size of the List + 1 : 1500 ms. newList = new ArrayList<Foo>(initialList.size() + 1); (as explained by @Soronthar)

Source : http://pastebin.com/c2C5c9Ha