Question

I was looking through the LinkedList API for Java 7 and noticed something curious. there does not appear to be a “remove before (or after)” type of method. For example, If I have a 100 element LinkedList, and I want to remove the first 20 elements, Java seems to force you to remove them one at a time, rather than moving the start pointer to the 21st element and deleting the link between 20 and 21. It seems like this is an operation that can be done in O(1) time, instead of O(n) time as Java seems to force you to do it.

Am I missing something here, or is it just a glaring hole in Java?

EDIT

I am aware of the sublist(int, int) method in the List interface. I still think that this will be slightly less efficient than the generic “chop off the first (or last) n” use case.

EDIT 2

Touche to everyone who pointed out that finding the nth element is not O(1). Regardless of the ease of chopping off the first n-1 elements, it still takes O(n) time to find the nth.

However, as Dilum Ranatunga points out, there is the possibility that an iterator already exists at the given position. In this case, it would still be useful to say “I am here, remove all before me”.

Answer 1

It’s still an O(n) operation no matter what you do.

You don’t have direct access to the individual nodes of the linked list, so you would have to traverse the list first to get access to the 21st node. Once you were there it would be O(1) to ‘re-head’ the list, but it’s still O(n) for the entire, atomic operation.