Why is ArrayDeque better than LinkedList

Linked structures are possibly the worst structure to iterate with a cache miss on each element. On top of it they consume way more memory.

If you need add/remove of the both ends, ArrayDeque is significantly better than a linked list. Random access each element is also O(1) for a cyclic queue.

The only better operation of a linked list is removing the current element during iteration.

I believe that the main performance bottleneck in LinkedList is the fact that whenever you push to any end of the deque, behind the scene the implementation allocates a new linked list node, which essentially involves JVM/OS, and that's expensive. Also, whenever you pop from any end, the internal nodes of LinkedList become eligible for garbage collection and that's more work behind the scene. Also, since the linked list nodes are allocated here and there, usage of CPU cache won't provide much benefit.

If it might be of interest, I have a proof that adding (appending) an element to ArrayList or ArrayDeque runs in amortized constant time; refer to this.

ArrayDeque is new with Java 6, which is why a lot of code (especially projects that try to be compatible with earlier Java versions) don't use it.

It's "better" in some cases because you're not allocating a node for each item to insert; instead all elements are stored in a giant array, which is resized if it gets full.