ArrayDeque vs ArrayList to implement a stack

Question

The documentation for ArrayDeque says:

This class is likely to be faster than Stack when used as a stack, and faster than LinkedList when used as a queue.

There is no mention of the difference between using an ArrayDeque as a stack and using an ArrayList. You can use an ArrayList as a stack as follows.

list.add(object);                      // push object = list.remove(list.size() - 1); // pop 

I found that when I used an ArrayList in this way only, its performance is worse than ArrayDeque. What accounts for this difference? Surely, it can't just be the calls to size()? Internally, both ArrayList and ArrayDeque are implemented using an Object[] that is replaced by a larger array when required, so surely the performance should be about the same?

Answer 1

The main difference between both implementation is the resize strategy

• ArrayList is resized to a new size of oldCapacity + (oldCapacity >> 1), resulting in an increse of ~50%. The default capacity is 10, resulting in a capacities after resize of 15, 22, 33, 49, 73, 109, 163, 244, 366...

• ArrayDeque is always resized to a power of 2. On resize, the capacity is doubled. Starting with the default of 16, the resuling capacities after resize are 32, 64, 128, 256,...

So the ArrayDeque reaches higher capacities with much less resize operation, which are costly because of array copy. I.e. to store 256 in default-sized ArrayList, it requires 9 resize calls, while the ArrayDeque only need 4. Array copy may be fast, but may also require a GC to free some space for the new data sets, in addition to the memory copy costs (where the ArrayDeque might also perform better due to it's alignment to power-of-2).

Both datastructures have best-case complexity of O(1) for push and pop through direct access on either head & tail (ArrayDequeue) respectively add and remove(Last) size (ArrayList)