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>();

you know about generics but not about implements/extends ? Seriously ? — Radu Murzea
– Radu Murzea, Commented May 6, 2012 at 14:09
@SoboLAN I guess he's in learning phase: "I quickly wrote a linked list class in Java", not using the generic LinkedList. — Luiggi Mendoza
– Luiggi Mendoza, Commented May 6, 2012 at 14:17
yes - see my post "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)" — Tim
– Tim, Commented May 6, 2012 at 14:34

Luiggi Mendoza · Accepted Answer · 2013-06-11 17:44:33Z

6

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.

edited Jun 11, 2013 at 17:44

answered May 6, 2012 at 14:04

Luiggi Mendoza

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3 Comments

Tim Over a year ago

hmm i think this is exactly what i am looking for.. going to test it out quickly, thanks.

Tim Over a year ago

so if i declare the queue as an interface. Eg: public interface Queue<T>{ // Methods} those methods must not have a function body but must simply have the same name as the relevant method in the LinkedList class?

Luiggi Mendoza Over a year ago

@Tim yes you're right. More info here and here

Tudor · Accepted Answer · 2012-05-06 13:59:01Z

3

Two mistakes in your code:

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

answered May 6, 2012 at 13:59

Tudor

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3 Comments

Tim Over a year ago

ok, so what can i do to make Queue<String> aQueue = new LinkedList<String>(); work? If I remove the implements linkedList from the queue class will this correct it? I am going to test it in the meanwhile... Ok i just tried it out... it says "incompatible types" ( which is why i put implements.. in in the first place)

Tim Over a year ago

cool that solves the one problem... thanks :)... the reason i wanted to have that last snippet was because in my one program i create some linked lists with objects of another class... and i want to be able to copy that linked list over to a queue

Tudor Over a year ago

@Tim: In the current form it's not possible. If you use the code I posted then there is no relationship between LinkedList and Queue so you cannot assign a LinkedList to a Queue. Furthermore, if you extend LinkedList it's still not correct because then Queue is a specialized linked list and so cannot appear to the left of the assignment.

Nagappa L M · Accepted Answer · 2014-06-17 13:23:36Z

-1

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

answered Jun 17, 2014 at 13:23

Nagappa L M

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