Queue using linked list implementation in Java

Question

I quickly wrote a linked list class in Java. I want to write another queue class which uses the linked list. How would I achieve this in Java? I don't fully understand the implements / extends keywords... this is what my queue looks like ( for example):

public class Queue<T> implements LinkedList
{
    protected LinkedList<T> list;

    public Queue() {
        list = new LinkedList<T>();
    }

    public void add( T element) {
        list.add( element);
    }

    public T removeLast() {
        return list.removeLast();
    }   
}

Also note that the linked list class is also generic. I know there are already built in classes to achieve this functionality, but I wanted to learn ( which is why I am trying to do this manually)

EDIT: Additionally, in the end, I would like to be able to say something like this:

Queue<String> aQueue = new LinkedList<String>();

Answer 1

If you want a behavior like Queue<String> aQueue = new LinkedList<String>(); then your LinkedList must extend/implement the Queue class/interface. Remember that a super class can be the object reference instance of a sub class, not viceversa.

public class LinkedList<T> implements Queue<T> {

    class Node<T> {
        T data;
        Node<T> next;
    }

    //all your behavior here
}

Also, as the Java documentation states, Queue is an interface and LinkedList implements it.

Note: If you want to implement a Queue using your LinkedList, you should see the code sample posted by @Tudor.

Answer 2

Since Linkedlist implements queue interface we can use poll, peek methods directly....no need of extra code

Answer 3

Two mistakes in your code:

1. You are both implementing LinkedList (did you mean extend?) and using composition by having a LinkedList inside your class.

2. This piece of code will not work: Queue<String> aQueue = new LinkedList<String>(); because according to point 1, Queue is either a subclass of LinkedList or contains a LinkedList, which makes your code incorrect.

In fact, the last code snippet makes very little sense. I assume that what you want to do is create a Queue that internally uses a linked list. In that case just use:

public class Queue<T>
{
    protected LinkedList<T> list;

    public Queue() {
        list = new LinkedList<T>();
    }

    public void add( T element) {
        list.add( element);
    }

    public T removeLast() {
        return list.removeLast();
    }   
}

And then:

Queue<String> queue = new Queue<String>();