Exception in thread "main" java.lang.ClassCastException: javafx.util.Pair cannot be cast to java.lang.Comparable

Also, this question had asked many times, I'm unable to figure that out according to my solution. I'm writing the Dijkstra algorithm for the weighted graph. I used ArrayList<ArrayList<Pair<Integer, Integer>>> for storing my weights and edges. While the implementation of the Dijkstra function, I used MinHeap which is PriorityQueue in java of type Pair<Integer, Integer>. At the time of adding pairs to the heap, I getting this error.

import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;
import javafx.util.Pair; 
public class Solution{
public static void dijkstra(ArrayList<ArrayList<Pair<Integer, Integer>>> graph, int V, int E, int S) {
int dist[] = new int[V+1];
Arrays.fill(dist, Integer.MAX_VALUE);
dist[S] = 0;
PriorityQueue< Pair<Integer, Integer> > heap = new PriorityQueue<>();
heap.add(new Pair<Integer, Integer>(0, S) );
while(heap.size() != 0) {
Pair<Integer, Integer> curr = heap.poll();
int w = curr.getKey(), x = curr.getValue();
System.out.println(x);
if(w <= dist[x]) {
for(Pair<Integer, Integer> temp : graph.get(x)){
int x2 = temp.getKey(), w2 = temp.getValue();
if(w+w2 < dist[x2]){
dist[x2] = w + w2;
//Facing the error in below line
heap.add(new Pair<Integer, Integer>(w+w2, x2));
}
}
}
}
for(int i = 1; i <= V; i++) {
if(i == S) continue;
if(dist[i] == Integer.MAX_VALUE)
System.out.print("-1 ");
else
System.out.print(dist[i]+" ");
}
}
//Adding edge to a graph
public static void addEdge( ArrayList<ArrayList<Pair<Integer, Integer>>> graph, int u, int v, int w){
graph.get(u).add(new Pair<Integer, Integer>(v, w));
graph.get(v).add(new Pair<Integer, Integer>(u, w));
}
//Main Function
public static void main(String args[]) {
Scanner in = new Scanner(System.in);
int t = in.nextInt();
while(t-- != 0) {
int n = in.nextInt();
int m = in.nextInt();
ArrayList<ArrayList<Pair<Integer, Integer>>> graph = new ArrayList<>();
for(int i = 0; i <= n; i++) {
graph.add(new ArrayList<Pair<Integer, Integer>>());
}
for(int i = 0; i < m; i++) {
addEdge(graph, in.nextInt(), in.nextInt(), in.nextInt());
}
int s = in.nextInt();
dijkstra(graph, n, m, s);
System.out.println();
}
}
}

I'm also adding the duplicates into the heaps as it doesn't affect my algorithm.

Because PriorityQueue maintains sorted orders of the objects it stores, And for that it need some comparison logic between objects.

In your case it is Pair, so the error is asking you to either make your Pair class implement Comparable interface or provide priority queue a comparator to do the sorting of pair object.

Have a look at this https://www.callicoder.com/java-priority-queue/

Basically you have to provide custom comparator.