Java Program to Implement Dijkstra’s Algorithm using Queue

This Java program,to Implement Dijkstra’s algorithm using 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.

Here is the source code of the Java program to implement Dijkstra’s algorithm using Queue. The Java program is successfully compiled and run on a Linux system. The program output is also shown below.

```
import java.util.HashSet;
```

import java.util.InputMismatchException;

```
import java.util.LinkedList;
```
```
import java.util.Queue;
```
```
import java.util.Scanner;
```
```
import java.util.Set;
```
```
 
```
```
public class DijkstraQueue
```
```
{
```
```
    private int distances[];
```
```
    private Queue<Integer> queue;
```
```
    private Set<Integer> settled;
```
```
    private int number_of_nodes;
```
```
    private int adjacencyMatrix[][];
```
```
 
```

    public DijkstraQueue(int number_of_nodes)

```
    {
```

        this.number_of_nodes = number_of_nodes;

        distances = new int[number_of_nodes + 1];

        settled = new HashSet<Integer>();

        queue = new LinkedList<Integer>();

        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;

```
        }
```
```
 
```
```
        queue.add(source);
```
```
        distances[source] = 0;
```
```
 
```
```
        while (!queue.isEmpty())
```
```
        {
```

            evaluationNode = getNodeWithMinimumDistanceFromQueue();

            settled.add(evaluationNode);

            evaluateNeighbours(evaluationNode);

```
        }
```
```
    }
```
```
 
```

    private int getNodeWithMinimumDistanceFromQueue()

```
    {
```
```
        int min ;
```
```
        int node = 0;
```

        Iterator<Integer> iterator = queue.iterator();

```
        node = iterator.next();
```
```
        min = distances[node];
```
```
 
```

        for (int i = 1; i <= distances.length; i++)

```
        {
```
```
            if (queue.contains(i))
```
```
            {
```

                if (distances[i] <= min)

```
                {
```
```
                    min = distances[i];
```
```
                    node = i;			
```
```
                }
```
```
            }
```
```
        }
```
```
        queue.remove(node);
```
```
        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;

```
                    }
```

                    queue.add(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();
```

            DijkstraQueue dijkstrasQueue = new DijkstraQueue(number_of_vertices);

            dijkstrasQueue.dijkstra_algorithm(adjacency_matrix, source);

```
 
```

            System.out.println("The Shorted Path to all nodes are ");

            for (int i = 1; i <= dijkstrasQueue.distances.length - 1; i++)

```
            {
```

                System.out.println(source + " to " + i + " is " + dijkstrasQueue.distances[i]);

```
            }
```

        } catch (InputMismatchException inputMismatch)

```
        {
```

            System.out.println("Wrong Input Format");

```
        }
```
```
        scan.close();
```
```
    }
```
```
}
```

$javac DijkstraQueue.java
$java DijkstraQueue 
Enter the number of vertices
5
Enter the Weighted Matrix for the graph
0 7 0 0 2
0 0 1 0 2
0 0 0 4 0
0 0 5 0 0
0 3 8 5 0
Enter the source 
1
The Shorted Path to all nodes are 
1 to 1 is 0
1 to 2 is 5
1 to 3 is 6
1 to 4 is 7
1 to 5 is 2

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