However, ArrayLists take up as much memory as is allocated for the capacity, regardless of whether elements have actually been added. Each element of a LinkedList has more overhead since pointers to the next and previous elements are also stored. Note that ArrayDeque may be a good alternative to LinkedList for adding and removing from the head, but it is not a List.Īlso, if you have large lists, keep in mind that memory usage is also different. On the other side, seeking in a LinkedList means following the links in O(n) ( n/2 steps) for worst case, whereas in an ArrayList the desired position can be computed mathematically and accessed in O(1).Īnother benefit of using a LinkedList arises when you add or remove from the head of the list, since those operations are O(1), while they are O(n) for ArrayList. In an array list, the remainder of the array needs to be moved (i.e. These operations can then be done in O(1) by changing the list locally only. The main benefits of using a LinkedList arise when you re-use existing iterators to insert and remove elements. (Iterating over an ArrayList is technically faster, but unless you're doing something really performance-sensitive, you shouldn't worry about this - they're both constants.) Iterating over either kind of List is practically equally cheap. So depending on the operations you intend to do, you should choose the implementations accordingly. Also, if you add more elements than the capacity of the underlying array, a new array (1.5 times the size) is allocated, and the old array is copied to the new one, so adding to an ArrayList is O(n) in the worst case but constant on average. But adding or removing from anywhere but the end requires shifting all the latter elements over, either to make an opening or fill the gap. Javadoc says "operations that index into the list will traverse the list from the beginning or the end, whichever is closer", so those methods are O(n) ( n/4 steps) on average, though O(1) for index = 0.ĪrrayList, on the other hand, allow fast random read access, so you can grab any element in constant time. In other words, you can walk the list forwards or backwards, but finding a position in the list takes time proportional to the size of the list. LinkedList allows for constant-time insertions or removals using iterators, but only sequential access of elements. Note: Many of the operations need n/2 steps on average, constant number of steps in the best case (end of list), n steps in the worst case (start of list) ListIterator.add(E element) is O(n) (with n/2 steps on average).Iterator.remove() is O(n) (with n/2 steps on average).remove(int index) is O(n) (with n/2 steps on average).add(int index, E element) is O(n) (with n/2 steps on average).add(E element) is O(1) amortized, but O(n) worst-case since the array must be resized and copied.index = 0), and n/2 steps in worst case (middle of list) Note: Many of the operations need n/4 steps on average, constant number of steps in the best case (e.g. remove(int index) is O(n) (with n/4 steps on average), but O(1) when index = 0 or index = list.size() - 1 (in this case, you can also use removeFirst() and removeLast()).
add(int index, E element) is O(n) (with n/4 steps on average), but O(1) when index = 0 or index = list.size() - 1 (in this case, you can also use addFirst() and addLast()/ add()).get(int index) is O(n) (with n/4 steps on average), but O(1) when index = 0 or index = list.size() - 1 (in this case, you can also use getFirst() and getLast()).
ArrayList implements it with a dynamically re-sizing array.Īs with standard linked list and array operations, the various methods will have different algorithmic runtimes. LinkedList implements it with a doubly-linked list. LinkedList and ArrayList are two different implementations of the List interface. In LinkedList adding an element takes O(n) time and accessing also takes O(n) time but LinkedList uses more memory than ArrayList. TLDR, in ArrayList accessing an element takes constant time and adding an element takes O(n) time. If you're not sure - just start with ArrayList. Summary ArrayList with ArrayDeque are preferable in many more use-cases than LinkedList.
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