I've always been one to simply use:
List<String> names = new ArrayList<>();
I use the interface as the type name for portability, so that when I ask questions such as this, I can rework my code.
When should LinkedList
be used over ArrayList
and vice-versa?
Best Answer
Summary
ArrayList
withArrayDeque
are preferable in many more use-cases thanLinkedList
. If you're not sure — just start withArrayList
.TLDR, in
ArrayList
accessing an element takes constant time [O(1)] and adding an element takes O(n) time [worst case]. InLinkedList
inserting an element takes O(n) time and accessing also takes O(n) time butLinkedList
uses more memory thanArrayList
.LinkedList
andArrayList
are two different implementations of theList
interface.LinkedList
implements it with a doubly-linked list.ArrayList
implements it with a dynamically re-sizing array.As with standard linked list and array operations, the various methods will have different algorithmic runtimes.
For
LinkedList<E>
get(int index)
is O(n) (with n/4 steps on average), but O(1) whenindex = 0
orindex = list.size() - 1
(in this case, you can also usegetFirst()
andgetLast()
). One of the main benefits ofLinkedList<E>
add(int index, E element)
is O(n) (with n/4 steps on average), but O(1) whenindex = 0
orindex = list.size() - 1
(in this case, you can also useaddFirst()
andaddLast()
/add()
). One of the main benefits ofLinkedList<E>
remove(int index)
is O(n) (with n/4 steps on average), but O(1) whenindex = 0
orindex = list.size() - 1
(in this case, you can also useremoveFirst()
andremoveLast()
). One of the main benefits ofLinkedList<E>
Iterator.remove()
is O(1). One of the main benefits ofLinkedList<E>
ListIterator.add(E element)
is O(1). One of the main benefits ofLinkedList<E>
Note: Many of the operations need n/4 steps on average, constant number of steps in the best case (e.g. index = 0), and n/2 steps in worst case (middle of list)
For
ArrayList<E>
get(int index)
is O(1). Main benefit ofArrayList<E>
add(E element)
is O(1) amortized, but O(n) worst-case since the array must be resized and copiedadd(int index, E element)
is O(n) (with n/2 steps on average)remove(int index)
is O(n) (with n/2 steps on average)Iterator.remove()
is O(n) (with n/2 steps on average)ListIterator.add(E element)
is O(n) (with n/2 steps on average)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)
LinkedList<E>
allows for constant-time insertions or removals using iterators, but only sequential access of elements. 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. 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) forindex = 0
.ArrayList<E>
, on the other hand, allow fast random read access, so you can grab any element in constant time. 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. 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 anArrayList
is O(n) in the worst case but constant on average.So depending on the operations you intend to do, you should choose the implementations accordingly. Iterating over either kind of List is practically equally cheap. (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.)The main benefits of using a
LinkedList
arise when you re-use existing iterators to insert and remove elements. These operations can then be done in O(1) by changing the list locally only. In an array list, the remainder of the array needs to be moved (i.e. copied). On the other side, seeking in aLinkedList
means following the links in O(n) (n/2 steps) for worst case, whereas in anArrayList
the desired position can be computed mathematically and accessed in O(1).Another 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) forArrayList
. Note thatArrayDeque
may be a good alternative toLinkedList
for adding and removing from the head, but it is not aList
.Also, if you have large lists, keep in mind that memory usage is also different. Each element of a
LinkedList
has more overhead since pointers to the next and previous elements are also stored.ArrayLists
don't have this overhead. However,ArrayLists
take up as much memory as is allocated for the capacity, regardless of whether elements have actually been added.The default initial capacity of an
ArrayList
is pretty small (10 from Java 1.4 - 1.8). But since the underlying implementation is an array, the array must be resized if you add a lot of elements. To avoid the high cost of resizing when you know you're going to add a lot of elements, construct theArrayList
with a higher initial capacity.If the data structures perspective is used to understand the two structures, a LinkedList is basically a sequential data structure which contains a head Node. The Node is a wrapper for two components : a value of type T [accepted through generics] and another reference to the Node linked to it. So, we can assert it is a recursive data structure (a Node contains another Node which has another Node and so on...). Addition of elements takes linear time in LinkedList as stated above.
An ArrayList is a growable array. It is just like a regular array. Under the hood, when an element is added, and the ArrayList is already full to capacity, it creates another array with a size which is greater than previous size. The elements are then copied from previous array to new one and the elements that are to be added are also placed at the specified indices.