key in java - priority queue

I am trying to learn java and I am stumped by this example- I can't find what the key is

public class OrderedArrayMaxPQ<Key extends Comparable<Key>> {
    private Key[] pq;          // elements

This is the example of the priority queue .. i got it from http://algs4.cs.princeton.edu/24pq/OrderedArrayMaxPQ.java.html I thought a priority queue should have a data + a priority value. I am not sure how these guys are doing it.

public class OrderedArrayMaxPQ<Key extends Comparable<Key>> {
    private Key[] pq;          // elements
    private int N;             // number of elements

    // set inititial size of heap to hold size elements
    public OrderedArrayMaxPQ(int capacity) {
        pq = (Key[]) (new Comparable[capacity]);
        N = 0;
    }


    public boolean isEmpty() { return N == 0;  }
    public int size()        { return N;       } 
    public Key delMax()      { return pq[--N开发者_如何学Go]; }

    public void insert(Key key) {
        int i = N-1;
        while (i >= 0 && less(key, pq[i])) {
            pq[i+1] = pq[i];
            i--;
        }
        pq[i+1] = key;
        N++;
    }

The priority is implemented within the Key implementation via its Comparable interface.

In the example you link to, they use String elements, so the priority is String's implementation of Comparable.

If you wanted to implement, say, a numeric priority, you'd create a priority queue with your class, which would implement Comparable, comparing the numeric value of the priority field.

You don't need Key[], just Comparable[].

Like the others say, focus on the comparable interface. A good way to get a feel for how java implements priority queues is to note the fact that String, Integer, Float, and other classes implement the Comparator interface.

This means that , by the objects definition, it is comparable (<,>, or =) to other objects of the same type.

I would suggest , rather than learning about priority queues, you

1) play with the sorting of simpler java data structures like Vectors and TreeMaps. 2) Look through the Collections API. Write a program which, for example, uses Collections.sort to sort some different lists.
3) Try to write an object which implements the Comparator interface, and sort a collection of these objects.