Question

I am trying to implement Dijkstra’s Algorithm for adjaceny matrix, and I am using Java priority Queue to do so.
For my vertex I am creating a custom comparator class, but I am getting the following error:

 <anonymous Dijkstra$1> is not abstract and does not override abstract method compare(vertex,vertex) in java.util.Comparator
        PriorityQueue<vertex> Q = new PriorityQueue<vertex>(n,new Comparator<vertex>() {        

here is the code:

import java.util.Scanner;
import java.io.File;
import java.util.PriorityQueue;
import java.util.Comparator;
class vertex {
    int v,d;
    public vertex(int num,int dis){
        v =num;
        d=dis;
    }
    public int getv(){
        return v;
    }
    public int getd(){
        return d;
    }
}

and then I use this to create a new priority queue:

PriorityQueue<vertex> Q = new PriorityQueue<vertex>(n,new Comparator<vertex>() {        
            public int compare (Object a, Object b){
            vertex v1 = (vertex)a;
            vertex v2 = (vertex)b;
            if (v1.getd() > v2.getd()){
                return +1;
            }else if (v1.getd() < v2.getd()){
                return -1;
            }else {
                return 0;
            }
        }});

Answer 1

the error you are getting is because you did’nt implemented the correct method. the signature of the compare method for Comparator<vertex> is public int compare (vertex a, vertex b); and not public int compare (Object a, Object b);.

moreover, as JB Nizer stated wisely, it would be better to implement your compare method as follows:

public int compare (vertex a, vertex b) {
    return Integer.compare(a.getd(), b.getd());
}