PriorityQueue Sort

Question

public class Flight implements Comparable {

....

public int compareTo(Object obj){
    Flight f = (Flight) obj;
    Integer i1 = (Integer) f.priority;
    Integer i2 = (Integer) priority;
    if(f == null)
        return 1;
    else 
        return i2.compareTo(i1);
}

....

public class JavaPriorityFlightQueue {


    public PriorityQueue flights;

....

public void joinQueue(Flight f){
        flights.add(f);
        Collections.sort(flights);
    }

.....

Exception in thread "main" java.lang.Error: Unresolved compilation problem: The method sort(List) in the type Collections is not applicable for the arguments (PriorityQueue)

at section3.JavaPriorityFlightQueue.joinQueue(JavaPriorityFlightQueue.java:31)
at section3.FlightTest003.main(FlightTest003.java:19)

I used the exact same compareTo for a LinkedList and it works, and everything is the same I have not missed something out (I think). I do not understand how it works for LinkedList but not PriorityQueue.

The next value from a PriorityQueue is always the lowest. If you constructor the PriorityQueue with the right Comparator you shouldn't need additional sorting. — Peter Lawrey
– Peter Lawrey, Commented Nov 12, 2012 at 12:22

PermGenError · Accepted Answer · 2012-11-12 12:22:55Z

3

Collections.sort(List<E>) only accepts List implementing classes. java.util.LinkedList implemnts List inteface, where as Priorityqueue doesnt implement List. Example:

PriorityQueue<String> pq = new PriorityQueue<String>();
        Collections.sort(pq);//compiler error here sort expects a List not priorityQueue

check Collections.sort(List) signature

one way to sort a priority queue using Sort method is to convert priorityqueue to Array and use Arrays.sort().

Arrays.sort(pq.toArray());

or use a constructor of PQ which takes Comparator as an second argument.

PriorityQueue pq = new PriorityQueue(initialcapacity, Comparator);

and read about java.util.Comparator

edited Nov 12, 2012 at 12:22

answered Nov 12, 2012 at 12:09

PermGenError

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4 Comments

user1817988 Over a year ago

I see so I cannot sort a Queue?

user1817988 Over a year ago

Comparator<? super E> comparator() Returns the comparator used to order the elements in this queue, or null if this queue is sorted according to the natural ordering of its elements.

user1817988 Over a year ago

This was in the priorityqueue doc, what is this Comparator exactly and what is it/can it be used for?

PermGenError Over a year ago

Comparator is an interface in java.util package which is used to custom sort. you could also use that constructor of the priority queue to sort it based on the comparator.

The Cat · Accepted Answer · 2012-11-12 12:12:51Z

0

Collections.sort can only take a list as an argument, which doesn't really make sense as it is in the Collections class. Sadly although PriorityQueue is a Collection, it does not implement List.

answered Nov 12, 2012 at 12:12

The Cat

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