Queue Interface using Java

[Jony27]

Hi all, have a little practical for one of my modules, Data Structures and Algorithm.

The problem

A queue is a data structure where new items enter at the back or rear, and items leave from the
front.In your textbook and in your lectures different implementations of a Queue have been discussed.The first part of this assessment is to implement a Csc2001Queue in a file Csc2001Queue.java which implements the CscQueueInterface. Download the interface CscQueueInterface.java from Queens online. Implement the eight methods described in the interface. You can use an array, a circular array,a java Linked List or the generic linked list you created in Practical 2 in your implementation.

It asks to implement the following methods:
offer (E item)
E poll
E remove
E peek
E element
size
printValues

I have the following code written, but when using a tester class to print the values, I dont get my expected outputs.

import java.util.AbstractQueue;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Queue;

public class Csc2001QueueInterface<E> extends AbstractQueue<E> implements Queue<E>{
	
	//Data field
	private LinkedList<E> theQueue = new LinkedList<E>();
	/**
     * Inserts an item at the rear of the queue.
     * @param item The element to add
     * @return true (always successful)
     */
	
	@Override
	public boolean offer(E item){
		theQueue.addLast(item);
		return true;
	}
	
	
	 /**
     * Removes the entry at the front of the queue and returns it.
     * @return The item removed if successful, or null if not
     */
	@Override
	public E poll(){
		if (size() == 0)
			return null;
			else 
				return theQueue.remove(0);
	}
	
	 /**
     * Removes the entry at the front of the queue and returns it.
     * @return The item removed if successful
	   @throws NoSuchElementException if the queue is empty
     */
	@Override
	public E remove(){
		if (size()==0)
			 throw new NoSuchElementException();
		else
			return theQueue.remove(0);
	}
	
	 /**
     * Returns the item at the front of the queue without removing it.
     * @return The item at the front if successful; null if not
     */
	@Override
	public E peek(){
		if (size()==0)
			return null;
		else return theQueue.getFirst();
	}
	
	   /**
     * Retrieves, but does not remove the entry at the front of the queue
     * and returns it.
     * @return The first entry in the queue if successful
     * @throws NoSuchElementException if this queue is empty
     */
	@Override
	public E element(){
		if (size() ==0)
			throw new NoSuchElementException();
		else
			return theQueue.get(0);
	}
	
	/**
     * Returns the number of elements in the queue
     * @return The number of elements in the queue
     */
	@Override
	public int size(){
		return theQueue.size();
	}
	
	/**
     * Returns true if this queue is empty
     * @return true if this queue is empty
     */
	@Override
	public boolean isEmpty() {
		return (theQueue == null);
	}
	
	 /**
     * Returns a String containing a concatenation of all the elements in this queue, leaving the queue unchanged
     * @return a String
     */

    public List<E> printValues(){
    	
    	List<E> result = new ArrayList<E>();
		if (!isEmpty())
		{
			result.add(theQueue.remove());
		}
		return result;
	
    }

Here is the tester class which was supposed to add the five names in the queue and print the values.

public class QueueTester {
	public static void main(String[] args)
	{
		Csc2001QueueInterface<String> queueList = new Csc2001QueueInterface<String>();
		queueList.offer("Jack");
		queueList.offer("Joe");
		queueList.offer("Jill");
		queueList.offer("Jane");
		queueList.offer("Jasper");
		
		System.out.println(queueList);
	}

}

When the tester class is ran, I get the following output

Csc2001QueueInterface@15db9742

Could someone kindle point me towards the right direction please.

Thanks.

 

[touch]

The queueList variable that you're trying to output is not something that can be represented in a text output (it's a complex data type).

You'll need to use one of the methods which the queue interface class has to output it's values in a text format. (peek? printValues?)

 

[Jony27]

When I changed the print method/line to

queueList.printValues();

It doesn't output anything, so I'm guessing it's how I implemented the offer() method or the printValues() that's the problem.

 

[touch]

When I changed the print method/line to

queueList.printValues();

It doesn't output anything, so I'm guessing it's how I implemented the offer() method or the printValues() that's the problem.

You'll still need the system.out.println part, just you need to give it some different data to output.
You can output the result of a method:

system.out.println(object.method());

 

[mid_gen]

Your printValues method is all wrong, doesn't even match the comment!

* Returns a String containing a concatenation of all the elements in this queue, leaving the queue unchanged
* @return a String

First up, you're returning an ArrayList, not a string.

You're only returning the first element of the queue.

You're changing the queue by removing the first element.

EDIT : your isEmpty function is wrong, it's doesn't return true if the queue is empty. It returns true if the queue is null, not the same thing.