Question

I have a LinkedList class, which serves as the base for a Queue class, which serves as the base for a PrintQueue class.

Here’s my PrintQueue class:

public class PrintQueue<T> {
    private Queue<T> queue;

    public PrintQueue() {
        queue = new Queue<T>();
    }

    public void lpr(String owner, int jobID) {
        queue.enqueue(new Job(owner, jobID));
    }
}

The queue.enqueue(...) line, three from the bottom is resulting in an error:

The method enqueue(T) in the type Queue is not applicable for the arguments (Job)

PrintQueue is a queue of Job objects.

The enqueue method in my Queue class looks like this:

    public void enqueue(T item) {
        queue.addToEnd(item);
    }

And the addToEnd method looks like this:

public void addToEnd(T item) {
    Node<T> itemnode = new Node<T>(item, null); 

    if (isEmpty()) {
        head = itemnode;
    }
    else {
        Node<T> curr = head;

        while (curr.getNext()!=null) {
            curr= curr.getNext();
        }

        curr.setNext(itemnode);
    }

    count++;

}

Why isn’t this working? All three classes (PrintQueue, Queue and LinkedList) are generic classes .

Answer 1

You’re trying to create a Queue<T> for some arbitrary type T – but then you’re trying to enqueue a Job.

What do you think it means to enqueue a Job on a Queue<String> for example?

Do you really need PrintQueue to be generic? I suspect you just want:

public class PrintQueue {
    private final Queue<Job> queue;

    public PrintQueue() {
        queue = new Queue<Job>();
    }

    public void lpr(String owner, int jobID) {
        queue.enqueue(new Job(owner, jobID));
    }
}

(Additionally, it’s not clear why you’ve got your own Queue class, when there are various good queues in the built-in class library… ones that don’t have O(N) “add to end” behaviour, too…)