Java – use Deque over Stack

data structuresdequejavastack

I need a Stack data structure for my use case. I should be able to push items into the data structure and I only want to retrieve the last item from the Stack. The JavaDoc for Stack says :

A more complete and consistent set of LIFO stack operations is
provided by the Deque interface and its implementations, which should
be used in preference to this class. For example:

Deque<Integer> stack = new ArrayDeque<>();

I definitely do not want synchronized behavior here as I will be using this datastructure local to a method . Apart from this why should I prefer Deque over Stack here ?

P.S: The javadoc from Deque says :

Deques can also be used as LIFO (Last-In-First-Out) stacks. This
interface should be used in preference to the legacy Stack class.

Best Answer

For one thing, it's more sensible in terms of inheritance. The fact that Stack extends Vector is really strange, in my view. Early in Java, inheritance was overused IMO - Properties being another example.

For me, the crucial word in the docs you quoted is consistent. Deque exposes a set of operations which is all about being able to fetch/add/remove items from the start or end of a collection, iterate etc - and that's it. There's deliberately no way to access an element by position, which Stack exposes because it's a subclass of Vector.

Oh, and also Stack has no interface, so if you know you need Stack operations you end up committing to a specific concrete class, which isn't usually a good idea.

Also as pointed out in the comments, Stack and Deque have reverse iteration orders:

Stack<Integer> stack = new Stack<>();
stack.push(1);
stack.push(2);
stack.push(3);
System.out.println(new ArrayList<>(stack)); // prints 1, 2, 3


Deque<Integer> deque = new ArrayDeque<>();
deque.push(1);
deque.push(2);
deque.push(3);
System.out.println(new ArrayList<>(deque)); // prints 3, 2, 1

which is also explained in the JavaDocs for Deque.iterator():

Returns an iterator over the elements in this deque in proper sequence. The elements will be returned in order from first (head) to last (tail).

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