programming-examples/java/Graph_Problems_Algorithms/Java Program to Implement Dijkstra’s Algorithm using Priority Queue.java

/*This Java program,to Implement Dijkstra’s algorithm using Priority Queue.Dijkstra’s algorithm is a graph search algorithm that solves the single-source shortest path problem for a graph with non-negative edge path costs, producing a shortest path tree.*/

import java.util.HashSet;
import java.util.InputMismatchException;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.Set;

public class DijkstraPriorityQueue
{
    private int distances[];
    private Set<Integer> settled;
    private PriorityQueue<Node> priorityQueue;
    private int number_of_nodes;
    private int adjacencyMatrix[][];

    public  DijkstraPriorityQueue(int number_of_nodes)
    {
        this.number_of_nodes = number_of_nodes;
        distances = new int[number_of_nodes + 1];
        settled = new HashSet<Integer>();
        priorityQueue = new PriorityQueue<Node>(number_of_nodes,new Node());
        adjacencyMatrix = new int[number_of_nodes + 1][number_of_nodes + 1];
    }

    public void dijkstra_algorithm(int adjacency_matrix[][], int source)
    {
        int evaluationNode;
        for (int i = 1; i <= number_of_nodes; i++)
            for (int j = 1; j <= number_of_nodes; j++)
                adjacencyMatrix[i][j] = adjacency_matrix[i][j];
        for (int i = 1; i <= number_of_nodes; i++)
            {
                distances[i] = Integer.MAX_VALUE;
            }
        priorityQueue.add(new Node(source, 0));
        distances[source] = 0;
        while (!priorityQueue.isEmpty())
            {
                evaluationNode = getNodeWithMinimumDistanceFromPriorityQueue();
                settled.add(evaluationNode);
                evaluateNeighbours(evaluationNode);
            }
    }

    private int getNodeWithMinimumDistanceFromPriorityQueue()
    {
        int node = priorityQueue.remove();
        return node;
    }

    private void evaluateNeighbours(int evaluationNode)
    {
        int edgeDistance = -1;
        int newDistance = -1;
        for (int destinationNode = 1; destinationNode <= number_of_nodes; destinationNode++)
            {
                if (!settled.contains(destinationNode))
                    {
                        if (adjacencyMatrix[evaluationNode][destinationNode] != Integer.MAX_VALUE)
                            {
                                edgeDistance = adjacencyMatrix[evaluationNode][destinationNode];
                                newDistance = distances[evaluationNode] + edgeDistance;
                                if (newDistance < distances[destinationNode])
                                    {
                                        distances[destinationNode] = newDistance;
                                    }
                                priorityQueue.add(new Node(destinationNode,distances[destinationNode]));
                            }
                    }
            }
    }

    public static void main(String... arg)
    {
        int adjacency_matrix[][];
        int number_of_vertices;
        int source = 0;
        Scanner scan = new Scanner(System.in);
        try
            {
                System.out.println("Enter the number of vertices");
                number_of_vertices = scan.nextInt();
                adjacency_matrix = new int[number_of_vertices + 1][number_of_vertices + 1];
                System.out.println("Enter the Weighted Matrix for the graph");
                for (int i = 1; i <= number_of_vertices; i++)
                    {
                        for (int j = 1; j <= number_of_vertices; j++)
                            {
                                adjacency_matrix[i][j] = scan.nextInt();
                                if (i == j)
                                    {
                                        adjacency_matrix[i][j] = 0;
                                        continue;
                                    }
                                if (adjacency_matrix[i][j] == 0)
                                    {
                                        adjacency_matrix[i][j] =  Integer.MAX_VALUE;
                                    }
                            }
                    }
                System.out.println("Enter the source ");
                source = scan.nextInt();
                DijkstraPriorityQueue dijkstrasPriorityQueue = new DijkstraPriorityQueue(number_of_vertices);
                dijkstrasPriorityQueue.dijkstra_algorithm(adjacency_matrix, source);
                System.out.println("The Shorted Path to all nodes are ");
                for (int i = 1; i <= dijkstrasPriorityQueue.distances.length - 1; i++)
                    {
                        System.out.println(source + " to " + i + " is " + dijkstrasPriorityQueue.distances[i]);
                    }
            }
        catch (InputMismatchException inputMismatch)
            {
                System.out.println("Wrong Input Format");
            }
        scan.close();
    }
}
class Node implements Comparator<Node>
{
    public int node;
    public int cost;

    public Node()
    {
    }

    public Node(int node, int cost)
    {
        this.node = node;
        this.cost = cost;
    }

    @Override
    public int compare(Node node1, Node node2)
    {
        if (node1.cost < node2.cost)
            return -1;
        if (node1.cost > node2.cost)
            return 1;
        return 0;
    }
}

/*

Enter the number of vertices
5
Enter the Weighted Matrix for the graph
0 9 6 5 3
0 0 0 0 0
0 2 0 4 0
0 0 0 0 0
0 0 0 0 0
Enter the source
1
The Shorted Path to all nodes are
1 to 1 is 0
1 to 2 is 8
1 to 3 is 6
1 to 4 is 5
1 to 5 is 3