# When to use LinkedList over ArrayList in Java?

`ArrayList`

is what you want. `LinkedList`

is almost always a (performance) bug.

Why `LinkedList`

sucks:

- It uses lots of small memory objects, and therefore impacts performance across the process.
- Lots of small objects are bad for cache-locality.
- Any indexed operation requires a traversal, i.e. has O(n) performance. This is not obvious in the source code, leading to algorithms O(n) slower than if
`ArrayList`

was used. - Getting good performance is tricky.
- Even when big-O performance is the same as
`ArrayList`

, it is probably going to be significantly slower anyway. - It's jarring to see
`LinkedList`

in source because it is probably the wrong choice.

**Summary** `ArrayList`

with `ArrayDeque`

are preferable in *many* more use-cases than `LinkedList`

. If you're not sure — just start with `ArrayList`

.

TLDR, in `ArrayList`

accessing an element takes constant time [O(1)] and adding an element takes O(n) time [worst case]. In `LinkedList`

inserting an element takes O(n) time and accessing also takes O(n) time but `LinkedList`

uses more memory than `ArrayList`

.

`LinkedList`

and `ArrayList`

are two different implementations of the `List`

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)*when`index = 0`

or`index = list.size() - 1`

(in this case, you can also use`getFirst()`

and`getLast()`

).**One of the main benefits of**`LinkedList<E>`

`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()`

).**One of the main benefits of**`LinkedList<E>`

`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()`

).**One of the main benefits of**`LinkedList<E>`

`Iterator.remove()`

is*O(1)*.**One of the main benefits of**`LinkedList<E>`

`ListIterator.add(E element)`

is*O(1)*.**One of the main benefits of**`LinkedList<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 of**`ArrayList<E>`

`add(E element)`

is*O(1)*amortized, but*O(n)*worst-case since the array must be resized and copied`add(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)* for `index = 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 an `ArrayList`

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 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)*.

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)* for `ArrayList`

. Note that `ArrayDeque`

may be a good alternative to `LinkedList`

for adding and removing from the head, but it is not a `List`

.

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 the `ArrayList`

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.

Thus far, nobody seems to have addressed the memory footprint of each of these lists besides the general consensus that a `LinkedList`

is "lots more" than an `ArrayList`

so I did some number crunching to demonstrate exactly how much both lists take up for N null references.

Since references are either 32 or 64 bits (even when null) on their relative systems, I have included 4 sets of data for 32 and 64 bit `LinkedLists`

and `ArrayLists`

.

**Note:** The sizes shown for the `ArrayList`

lines are for *trimmed lists* - In practice, the capacity of the backing array in an `ArrayList`

is generally larger than its current element count.

**Note 2:** *(thanks BeeOnRope)* As CompressedOops is default now from mid JDK6 and up, the values below for 64-bit machines will basically match their 32-bit counterparts, unless of course you specifically turn it off.

The result clearly shows that `LinkedList`

is a whole lot more than `ArrayList`

, especially with a very high element count. If memory is a factor, steer clear of `LinkedLists`

.

The formulas I used follow, let me know if I have done anything wrong and I will fix it up. 'b' is either 4 or 8 for 32 or 64 bit systems, and 'n' is the number of elements. Note the reason for the mods is because all objects in java will take up a multiple of 8 bytes space regardless of whether it is all used or not.

**ArrayList:**

`ArrayList object header + size integer + modCount integer + array reference + (array oject header + b * n) + MOD(array oject, 8) + MOD(ArrayList object, 8) == 8 + 4 + 4 + b + (12 + b * n) + MOD(12 + b * n, 8) + MOD(8 + 4 + 4 + b + (12 + b * n) + MOD(12 + b * n, 8), 8)`

**LinkedList:**

`LinkedList object header + size integer + modCount integer + reference to header + reference to footer + (node object overhead + reference to previous element + reference to next element + reference to element) * n) + MOD(node object, 8) * n + MOD(LinkedList object, 8) == 8 + 4 + 4 + 2 * b + (8 + 3 * b) * n + MOD(8 + 3 * b, 8) * n + MOD(8 + 4 + 4 + 2 * b + (8 + 3 * b) * n + MOD(8 + 3 * b, 8) * n, 8)`

```
Algorithm ArrayList LinkedList
seek front O(1) O(1)
seek back O(1) O(1)
seek to index O(1) O(N)
insert at front O(N) O(1)
insert at back O(1) O(1)
insert after an item O(N) O(1)
```

Algorithms: Big-Oh Notation (archived)

ArrayLists are good for write-once-read-many or appenders, but bad at add/remove from the front or middle.