When to use LinkedList and ArrayList in Java

Question

I want to know when to use LinkedList and when to use ArrayList

Answer 1

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 backward, 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 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.

Answer 2

ArrayList is what you want. LinkedList is almost always a (performance) bug.

LinkedList is almost the wrong choice, performance-wise. There are some very specific algorithms where a LinkedList is called for, but those are very, very rare and the algorithm will usually specifically depend on LinkedList's ability to insert and delete elements in the middle of the list relatively quickly, once you've navigated there with a ListIterator.

There is one common use case in which LinkedList outperforms ArrayList: that of a queue. However, if your goal is performance, instead of LinkedList you should also consider using an ArrayBlockingQueue (if you can determine an upper bound on your queue size ahead of time, and can afford to allocate all the memory up front).