Truncating a list to a given number of elements in java

Question

There is any method to truncate a list in java, for example, to the first 100 elements, discarding the others? (I mean, without iterating and/or copying/deleting elements one by one)

Answer 1

list.subList(100, list.size()).clear();

or:

list.subList(0, 100);

Answer 2

Try

List<String> items = Arrays.asList("1", "2", "3");
List<String> subItems = items.subList(0, 2);

You should bear in mind that subList returns a view of the items, so if you want the rest of the list to be eligible for garbage collection, you should copy the items you want to a new List:

List<String> subItems = new List<String>(items.subList(0, 2));

Answer 3

You did not specify the type of list. If it is an ArrayList, then

list.removeRange(100, list.size());

has cost O(1).

On the other hand, if it is a LinkedList, then removeRange will require O(list.size()) calls, and so clear() (it calls removeRange()). There appears to be no O(1) way to truncate a linked list in java, except for writing your own implementation. If the list is short, copying and losing the old reference, as suggesting by Ben Lings, is the fastest way to get what you want.