Implementing a queue in Java

Question

I am practicing with stacks and queues and have some questions about them (mostly on queues)

How would I implement a queue in my code?

package *****;

import java.util.*;

public class stackPractice {

    /**
    * @param args
    */
    public static void main(String[] args) {
        Stack st = new Stack();
        Queue q = new Queue();

        st.push(100);
        st.push(90);
        st.push(70);

        System.out.println(st);

        //st.pop();

        System.out.println(st.pop());
        System.out.println(st);
        System.out.println(st.peek());

        //value = st.peek();
    }

}

I got Stack st to work as a stack, but Queue is giving me problems

on the 2nd Queue after new, there is a red squiggly line that says "Cannot instantiate the type Queue".

Queue q = new *Queue*();

I am not sure what that means.

---edit---

I know there is no actual code for the queue to do anything yet (enqueue, dequeue, etc...).

You are probably referring to [the built-in Queue interface](http://docs.oracle.com/javase/7/docs/api/java/util/Queue.html) instead of your own implementation. Fix your imports. — Guillaume Poussel, Oct 23 '13 at 17:36

score 1 · Answer 1 · answered Oct 23 '13 at 17:36

1

Stack is a class in Java, but Queue is an interface, so you can't instantiate it. You'll need to call the constructor of one of its implementing classes.

answered Oct 23 '13 at 17:36

Bill the Lizard

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Reimeus · Answer 2 · 2013-10-23T17:48:08.590

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Queue is an interface and interfaces cannot be instantiated directly. Use one of its implementations to create an instance of the interface

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

edited Oct 23 '13 at 17:48

answered Oct 23 '13 at 17:36

Reimeus

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score 0 · Answer 3 · edited May 23 '17 at 11:57

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Queue is an interface so you cannot instantiate it but you need to "implement" it.

See http://docs.oracle.com/javase/7/docs/api/java/util/Queue.html

Duplicate here: How do I instantiate a Queue object in java?

edited May 23 '17 at 11:57

Community

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answered Oct 23 '13 at 17:37

Fabien Sa

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score 0 · Answer 4 · answered Oct 23 '13 at 17:38

Queue is an interface and can not be instanciated.

You could use a LinkedList. or one of the listened:

   AbstractQueue, ArrayBlockingQueue, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingQueue, LinkedList, PriorityBlockingQueue, PriorityQueue, SynchronousQueue

Implementing a queue in Java

4 Answers4