Question

I have a class inside the main class named Prims and a priority queue of the class type.After I created an instance of the class with its construct, I want to push the object into the queue.Compiled well, But it shows run-time error with NullPointerException .Here is the code:

package mst.prims;

import java.util.*;

    public class Main {

        /**
         * Minimum Spanning Tree - Prim's Algorithm
         * @author Kaidul
         */
        static final int MAX = 100; 

        static class Prims{
            int u, v, cost;
            Prims(int u, int v, int cost){
                this.u = u;
                this.v = v;
                this.cost = cost;
            }
        }

        static PriorityQueue<Prims> q, q1, q2 = new PriorityQueue<Prims>(MAX);

        public static void main(String[] args)  {


            Scanner input = new Scanner(System.in);

            for (int i = 0; i < 7; i++) {
                int u, v, cost;
                u = input.nextInt();
                v = input.nextInt();
                cost = input.nextInt();
                Prims temp = new Prims(u, v, cost);

                q.add(temp);
            }
        }

    }

Error :

1 2
2 3
Exception in thread "main" java.lang.NullPointerException
    at mst.prims.Main.main(Main.java:36)

I am new in Java and can’t fix it.

Answer 1

From the Javadoc for PriorityQueue:

A priority queue relying on natural ordering also does not permit insertion of non-comparable objects (doing so may result in ClassCastException).

You must implement Comparable in Prims. If you intend to use Prims in collections you should also override equals() and hashCode() with implementations appropriate for your object.