Please help!

[Betty7]

Hi,
I'm a beginner in Java programming. Sorry for any silly mistakes I'm trying to do a uniform cost search program on a graph but I have some problems in the code.
I'm not sure if my algorithm works correctly or not.
This is the problem I get:

Exception in thread "main" java.lang.ClassCastException: Edge cannot be cast to java.lang.Comparable
at java.util.PriorityQueue.siftUpComparable(PriorityQ ueue.java:578)
at java.util.PriorityQueue.siftUp(PriorityQueue.java: 574)
at java.util.PriorityQueue.offer(PriorityQueue.java:2 74)
at java.util.PriorityQueue.add(PriorityQueue.java:251 )
at UCS.UnifromCostSearch(UCS.java:86)
at UCS.main(UCS.java:115)

This is my program:

import java.util.*;

class Vertex 
{
	public String name;
	public Vertex Parent;
	public Edge[] adjacencies;
	public int weight;
	public Vertex vertex;

		 
	public Vertex(String vertexName)
	{
		name = vertexName;
	}
			
	public int getWeight()
	{
		return weight;
	}
	
	public void setWeight(int vertexWeight )
	{
		weight = vertexWeight;
	}
	
		
	public Edge[] getAdjacencies()
	{
		return adjacencies;
	}
	
	public void setAdjacencies(Edge[] adjacencies)
	{
		this.adjacencies = adjacencies;
	}
}

 
class Edge
{
	public int weight;
	public Vertex vertex;

	
	
	public Edge(Vertex vertexTarg, int vertexWeight)
	{
		vertex = vertexTarg;
		weight = vertexWeight;
	}
	
}


public class UCS
{
	
	public static void UnifromCostSearch(Vertex Start, Vertex Goal)
	{
		PriorityQueue Fringe = new PriorityQueue();
		ArrayList explored = new ArrayList();
		Fringe.add(Start);
		int cost =0;
			
		Vertex goal = Goal;
	
		while(!Fringe.isEmpty())
		{
			Vertex Parent = (Vertex)Fringe.poll();
		
			cost += Parent.getWeight();
		
			if(Parent == goal)
		    {
				System.out.println("Goal is found: " + goal);
				break;
			}
		
			explored.add(Parent);	        	      
	       
	    	Edge[] children = Parent.getAdjacencies();
				
			for(int i =0; i < children.length;i++)
			{
				Fringe.add(children[i]);
			}
						
		}
	}
	 

public static void main(String[] args)
{
    Vertex v0 = new Vertex("Redvile");
	Vertex v1 = new Vertex("Blueville");
	Vertex v2 = new Vertex("Greenville");
	Vertex v3 = new Vertex("Orangeville");
	Vertex v4 = new Vertex("Purpleville");

	v0.adjacencies = new Edge[]{ new Edge(v1, 5),
	                             new Edge(v2, 10),
                                 new Edge(v3, 8) };
	v1.adjacencies = new Edge[]{ new Edge(v0, 5),
	                             new Edge(v2, 3),
	                             new Edge(v4, 7) };
	v2.adjacencies = new Edge[]{ new Edge(v0, 10),
                                 new Edge(v1, 3) };
	v3.adjacencies = new Edge[]{ new Edge(v0, 8),
	                             new Edge(v4, 2) };
	v4.adjacencies = new Edge[]{ new Edge(v1, 7),
                                 new Edge(v3, 2) };
	Vertex[] vertices = { v0, v1, v2, v3, v4 };
	
    UnifromCostSearch(v0, v4);
   
}
}

Any help is greatly appreciated

[JosAH]

Have you read the API documentation for the PriorityQueue class? It needs its elements to have an order; i.e. for elements A and B it needs to decide whether A < B or A > B or A == B; that makes sense for a priority queue ... The elements need to implement the Comparable interface and your Edge class doesn't do that so your Java virtual machine complains.

kind regards,

Jos