Question

I was reading http://www.docjar.com/html/api/java/util/LinkedList.java.html

When you declare a queue in Java

    Queue<Integer> queue = new LinkedList<Integer>();

What happens behind the scene ? because I see queue is an interface with just method signatures, and LinkedList doesn’t directly implement it, so how does it override those methods (add(), peek(), poll(), offer(), and remove() ) and do the polymorphism like that ? I mean you can only access some certain methods but not all of them from LinkedList for example public void add(int index, E element) is no longer available as it makes the apparent type to Queue. Also didn’t we need to cast it ?

Answer 1

From the source code of the JDK:

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{

So LinkedList<E> doesn’t directly implement Queue<E>, but it does implement Deque<E>, which extends Queue<E>:

public interface Deque<E> extends Queue<E> {

Threfore, LinkedList<E> inherits the abstract methods of Queue<E>.

The overriding methods are defined directly in LinkedList<E> – as usual.