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.
Remember that big-O complexity describes asymptotic behaviour and may not reflect actual implementation speed. It describes how the cost of each operation grows with the size of the list, not the speed of each operation. For example, the following implementation of add
is O(1) but is not fast:
public class MyList extends LinkedList {
public void add(Object o) {
Thread.sleep(10000);
super.add(o);
}
}
I suspect in your case ArrayList is performing well because it increases it's internal buffer size fairly aggressively so there will not be a large number of reallocations. When the buffer does not need to be resized ArrayList will have faster add
s.
You also need to be very careful when you do this kind of profiling. I'd suggest you change your profiling code to do a warm-up phase (so the JIT has the opportunity to do some optimization without affecting your results) and average the results over a number of runs.
private final static int WARMUP = 1000;
private final static int TEST = 1000;
private final static int SIZE = 500000;
public void perfTest() {
// Warmup
for (int i = 0; i < WARMUP; ++i) {
buildArrayList();
}
// Test
long sum = 0;
for (int i = 0; i < TEST; ++i) {
sum += buildArrayList();
}
System.out.println("Average time to build array list: " + (sum / TEST));
}
public long buildArrayList() {
long start = System.nanoTime();
ArrayList a = new ArrayList();
for (int i = 0; i < SIZE; ++i) {
a.add(i);
}
long end = System.nanoTime();
return end - start;
}
... same for buildLinkedList
(Note that sum
may overflow and you might be better to use System.currentTimeMillis()
).
It's also possible that the compiler is optimizing away your empty get
loops. Make sure the loop actually does something to ensure that the right code is getting called.
Best Answer
LinkedList
might allocate fewer entries, but those entries are astronomically more expensive than they'd be forArrayList
-- enough that even the worst-caseArrayList
is cheaper as far as memory is concerned.(FYI, I think you've got it wrong;
ArrayList
grows by 1.5x when it's full, not 2x.)See e.g. https://github.com/DimitrisAndreou/memory-measurer/blob/master/ElementCostInDataStructures.txt :
LinkedList
consumes 24 bytes per element, whileArrayList
consumes in the best case 4 bytes per element, and in the worst case 6 bytes per element. (Results may vary depending on 32-bit versus 64-bit JVMs, and compressed object pointer options, but in those comparisonsLinkedList
costs at least 36 bytes/element, andArrayList
is at best 8 and at worst 12.)UPDATE:
To be clear, even in the worst case,
ArrayList
is 4x smaller than aLinkedList
with the same elements. The only possible way to makeLinkedList
win is to deliberately fix the comparison by callingensureCapacity
with a deliberately inflated value, or to remove lots of values from theArrayList
after they've been added.In short, it's basically impossible to make
LinkedList
win the memory comparison, and if you care about space, then callingtrimToSize()
on theArrayList
will instantly makeArrayList
win again by a huge margin. Seriously.ArrayList
wins.