Question

For my data structure class, I am trying to write a program that simulates a car wash and I want to give fancy cars a higher priority than regular ones using a priority queue. The problem I am having has something to do with Java not being able to type cast “Object” as an “ArrayQueue” (a simple FIFO implementation). What am I doing wrong and how can I fix it?

public class PriorityQueue<E>
{

    private ArrayQueue<E>[] queues;
    private int highest=0;
    private int manyItems=0;


    public PriorityQueue(int h)
    {
        highest=h;
        queues = (ArrayQueue<E>[]) new Object[highest+1];   <----problem is here
    }


    public void add(E item, int priority)
    {
        queues[priority].add(item);
        manyItems++;
    }


    public boolean isEmpty( )
    {
        return (manyItems == 0);
    }


    public E remove()
    {
        E answer=null;
        int counter=0;

        do
        {
            if(!queues[highest-counter].isEmpty())
            {
                answer = queues[highest-counter].remove();
                counter=highest+1;
            }
            else
                counter++;
        }while(highest-counter>=0);

        return answer;
    }
}

EDIT

Thank you both for the quick answer to this question. I solved the problem by following your advice and one other bit of code:

public PriorityQueue(int h)
{
    highest=h;
    queues = new ArrayQueue[highest+1];
    for(int i = 0; i <= highest; i++)
    {
        queues[i] = new ArrayQueue();
    }
}

Answer 1

An Object is an Object and (in most cases) not an ArrayQueue. So indeed the cast is not possible.

Creation of generic arrays is a problem too, but in your case, this should work:

public PriorityQueue(int h)
{
    highest=h;
    queues = new ArrayQueue[highest+1];   // Gives an ignorable warning
}

EDIT

The way it is explained in your textbook is incorrect, the book needs a new revision cycle 😉 The suggested cast is not allowed in Java, it’s like an attempt to do

String forEverUseless = (String) new Object(); // this will not give an empty String
                                               // but an ouch-that-hurts-Exception

which is more obvious. You can never cast a class to one of its subtypes (derived classes). This is true for all classes, including arrays and generic classes.

EDIT 2

Two more suggestions:

1. The ‘add’ method should get a check whether ‘priority’ is in the valid range of priorities, otherwise add will throw an exception (like in: queue.add(entry, -1))
2. A remove method usually has an argument – you might want to call it with the element that shall be removed from the queue. (Or – if you’re intention is something else, i suggest using the common queue operation names pop, push and peek)