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 inserting 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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