When to use queue over arraylist

Question

One basic argument to use a Queue over an ArrayList is that Queue guarantees FIFO behavior.

But if I add 10 elements to an ArrayList and then iterate over the elements starting from the 0th element, then I will retrieve the elements in the same order as they were added. So essentially, that guarantees a FIFO behavior.

What is so special about Queue as compared to traditional ArrayList?

Answer 1

You can look at the javadoc here. The main difference is a List lets you look at any element whenever you want. A queue only lets you look at the "next" one.

Think about it as a real queue or as a line for the cash register at a grocery store. You don't ask the guy in the middle or the end to pay next, you always ask the guy who's in the front/been waiting the longest.

It's worth noting that some lists are queues. Look at LinkedList, for example.

Answer 2

If I gave you a Queue instance then you would know that by iteratively calling remove() you would retrieve the elements in FIFO order. If i gave you an ArrayList instance then you can make no such guarantee.

Take the following code as an example:

        ArrayList<Integer> list = new ArrayList<Integer>();     list.add(5);     list.add(4);     list.add(3);     list.add(2);     list.add(1);       list.set(4,5);     list.set(3,4);     list.set(2,3);     list.set(1,2);     list.set(0,1);      System.out.println(list); 

If I were now to give you this list, then my iterating from 0 to 4 you would not get the elements in FIFO order.

Also, I would say another difference is abstraction. With a Queue instance you don't have to worry about indexes and this makes things easier to think about if you don't need everything ArrayList has to offer.