programming-examples/java/Graph_Problems_Algorithms/Java Program to Check the Connectivity of Graph Using BFS.java

//This is a java program to check the nodes are connected using BFS
import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;

public class Connectivity_BFS
{
    private final int      vertices;
    private int[][]        adjacency_matrix;
    private Queue<Integer> queue;

    public Connectivity_BFS(int v)
    {
        vertices = v;
        adjacency_matrix = new int[vertices + 1][vertices + 1];
        queue = new LinkedList<Integer>();
    }

    public void makeEdge(int to, int from, int edge)
    {
        try
            {
                adjacency_matrix[to][from] = edge;
                adjacency_matrix[from][to] = edge;
            }
        catch (ArrayIndexOutOfBoundsException index)
            {
                System.out.println("The vertices does not exists");
            }
    }

    public int getEdge(int to, int from)
    {
        try
            {
                return adjacency_matrix[to][from];
            }
        catch (ArrayIndexOutOfBoundsException index)
            {
                System.out.println("The vertices does not exists");
            }
        return -1;
    }

    public void bfs(int source)
    {
        int number_of_nodes = adjacency_matrix[source].length - 1;
        int[] visited = new int[number_of_nodes + 1];
        int i, element;
        visited[source] = 1;
        queue.add(source);
        while (!queue.isEmpty())
            {
                element = queue.remove();
                i = 1;// element;
                while (i <= number_of_nodes)
                    {
                        if (adjacency_matrix[element][i] == 1 && visited[i] == 0)
                            {
                                queue.add(i);
                                visited[i] = 1;
                            }
                        i++;
                    }
            }
        System.out.print("The source node " + source + " is connected to: ");
        int count = 0;
        for (int v = 1; v <= number_of_nodes; v++)
            if (visited[v] == 1)
                {
                    System.out.print(v + " ");
                    count++;
                }
        if (count == number_of_nodes)
            System.out.print("\nThe Graph is Connected ");
        else
            System.out.print("\nThe Graph is Disconnected ");
    }

    public static void main(String args[])
    {
        int v, e, count = 1, to = 0, from = 0;
        Scanner sc = new Scanner(System.in);
        Connectivity_BFS graph;
        System.out.println("The Undirected Graph Connectivity Test");
        try
            {
                System.out.println("Enter the number of vertices: ");
                v = sc.nextInt();
                System.out.println("Enter the number of edges: ");
                e = sc.nextInt();
                graph = new Connectivity_BFS(v);
                System.out.println("Enter the edges: <to> <from>");
                while (count <= e)
                    {
                        to = sc.nextInt();
                        from = sc.nextInt();
                        graph.makeEdge(to, from, 1);
                        count++;
                    }
                System.out.println("The adjacency matrix for the given graph is: ");
                System.out.print("  ");
                for (int i = 1; i <= v; i++)
                    System.out.print(i + " ");
                System.out.println();
                for (int i = 1; i <= v; i++)
                    {
                        System.out.print(i + " ");
                        for (int j = 1; j <= v; j++)
                            System.out.print(graph.getEdge(i, j) + " ");
                        System.out.println();
                    }
                System.out.println("Enter the Source Node: ");
                int sourceNode = sc.nextInt();
                graph.bfs(sourceNode);
            }
        catch (Exception E)
            {
                System.out.println("Something went wrong");
            }
        sc.close();
    }
}

/*
The Undirected Graph Connectivity Test(BFS)
Enter the number of vertices:
4
Enter the number of edges:
2
Enter the edges: <to> <from>
1 2
3 4
The adjacency matrix for the given graph is:
  1 2 3 4
1 0 1 0 0
2 1 0 0 0
3 0 0 0 1
4 0 0 1 0
Enter the Source Node:
3
The source node 3 is connected to: 3 4
The Graph is Disconnected

The Undirected Graph Connectivity Test(BFS)
Enter the number of vertices:
4
Enter the number of edges:
5
Enter the edges: <to> <from>
1 2
2 3
3 4
1 4
1 3
The adjacency matrix for the given graph is:
  1 2 3 4
1 0 1 1 1
2 1 0 1 0
3 1 1 0 1
4 1 0 1 0
Enter the Source Node:
4
The source node 4 is connected to: 1 2 3 4
The Graph is Connected