Question

I have a class that uses priority queue to display 5 strings in ascending order. I understand that to make it in descending order i can use the “collections.reverseOrder()” method. How use this method with the following code?

import java.util.*;
public class queue {

    public static void main (String[] args) {

        PriorityQueue<String> sQ = new PriorityQueue<String>();

        sQ.add("theodore");
        sQ.add("theo");
        sQ.add("Shailee");
        sQ.add("Deborah");
        sQ.add("Fernando");
        sQ.add("th");

        while (sQ.size() > 0)
            System.out.println(sQ.remove());

        Collections.reverseOrder(); //I am stuck here...
    }
}

Answer 1

Try something like this, before removing the elements in sQ:

PriorityQueue<String> reversed =
    new PriorityQueue<String>(sQ.size(), new Comparator<String>() {
    @Override
    public int compare(String o1, String o2) {
        return -o1.compareTo(o2);
    }
});
reversed.addAll(sQ); // now `reversed` contains the reversed priority queue

Because you’re using the natural ordering of String, it makes sense to just build another PriorityQueue passing as a parameter a new comparator that compares strings but reversing the order (notice the - sign in front of the comparison).

EDIT:

As has been pointed in the comments, this is an even simpler solution:

PriorityQueue<String> reversed =
    new PriorityQueue<String>(sQ.size(), Collections.reverseOrder());
reversed.addAll(sQ);