ArrayDeque has methods for stacks and queues. Mostly used stack and queue methods are the ones in the below:
Stack methods: push/poll/peek
Queue methods: push/poll/peek
The thing that I've done in the below code block is that, I tried to understand behavior of the ArrayDeque when both offer, push and add methods are used in the same object. The code that I wrote and its output is given below. What is the behavior of the ArrayDeque after calling push() method, which it assumes itself as a stack, and then calling the offer() method, which is stated as queue methods.
Deque<Integer> arrayDeque = new ArrayDeque<>();
arrayDeque.add(3);
arrayDeque.push(4);
arrayDeque.offer(6);
arrayDeque.addFirst(2);
arrayDeque.addLast(5);
arrayDeque.addFirst(1);
System.out.println("ArrayDeque: " + arrayDeque.toString());
The output is:
ArrayDeque: [1, 2, 4, 3, 6, 5]
Here is what it does step by step
// Add 3 at the tail of this deque
arrayDeque.add(3); -> [3]
// Add 4 at the head of this deque
arrayDeque.push(4); -> [4, 3]
// Add 6 at the tail of this deque
arrayDeque.offer(6); -> [4, 3, 6]
// Add 2 at the head of this deque
arrayDeque.addFirst(2); -> [2, 4, 3, 6]
// Add 5 at the tail of this deque
arrayDeque.addLast(5); -> [2, 4, 3, 6, 5]
// Add 1 at the head of this deque
arrayDeque.addFirst(1); -> [1, 2, 4, 3, 6, 5]
Keep in mind that the main purpose of a Deque unlike a Queue or a Stack is to have the ability to access/add the elements at/to both ends (head and tail).