Queue<Integer> q = new LinkedList<Integer>()
data-structures, java, queue
Solution
To explain with a (perhaps somewhat flawed) metaphor - think of a `LinkedList` as a piece of paper. Think of the assignment to a `Queue` as covering all but a little part of this piece of paper which reveals that it's a `Queue`.
If you call a `Queue` method on it, it will still do things like a `LinkedList` normally would, since that that's what the piece of paper is.
Given that most of it is covered, you can't see it's a `LinkedList`, all you can see is that it's a `Queue`, so you can only call the `Queue` methods on it.
The whole piece of paper is still there - you could simply remove the covering, which would be synonymous to casting it back to a `LinkedList`, which would then allow you to call any `LinkedList` method on it again.
Now for a bit more technical details:
By the way a `LinkedList` "normally would" do things I mentioned above, I mean a `LinkedList` is always a linked-list, even if you assign it to a `Queue` - it doesn't suddenly start using an array as the underlying implementation, as an `ArrayDeque` would (which also implements `Queue`), or something else.
`Queue` doesn't actually need to be FIFO (see the docs) (if it needed to be, a `LinkedList` would also need to be), so `LinkedList` would have quite a bit of freedom in this regard, so let's continue this explanation using `Deque` - it has methods to support addition and removal from either the front or the back.
Because `LinkedList` implements `Deque`, it needs to implement the `addFirst` function. Based on the docs, the `addFirst` function needs to add to the front of the `Deque`, and indeed, with a `LinkedList`, it will add to the front of the `LinkedList` (while the front of the `LinkedList` doesn't need to be the front of the `Deque`, by looking at the `Deque` methods implemented in `LinkedList`, we see that the front of the `Deque` is the front of the `LinkedList`, and all the methods add to / remove from one of the sides, do so from the correct side).
Now for an important albeit somewhat confusing note - `LinkedList` can, for example, implement `Deque` and have an `addFirst` that doesn't do what it's supposed to - it can just, for example, print some random text. There's nothing in the language itself that prevents this - since, as far as the compiler is concerned, implementing `Deque` just requires that you define a bunch of methods - there's no enforcement of what these methods are supposed to do. In terms of the Java API, and any decent library, it should be safe to assume that every class implementing an interface will conform to what that interface claims to do, but just keep in mind that there isn't something stopping it from not conforming when it comes to more shady libraries or less experienced programmers.
Problem
Here is an answer to "How do I instantiate a Queue object in java?", `Queue` is an interface. You can't instantiate an interface directly except via an anonymous inner class. Typically this isn't what you want to do for a collection. Instead, choose an existing implementation. For example: Queue q = new LinkedList(); or Queue q = new ArrayDeque(); Typically you pick a collection implementation by the performance and concurrency characteristics you're interested in. How does this work? What does it mean for Queue, an interface, to take on the implementation of LinkedList? Does that mean objects can be dequeued in a FIFO (first-in-first-out order), and calling linked list methods on `q` would work?