Java Program to Describe the Representation of Graph using Adjacency List

Code:

import java.util.HashMap;
import java.util.InputMismatchException;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Scanner;

public class GraphAdjacencyList 
{
   /* Makes use of Map collection to store the adjacency list for each vertex.*/
    private  Map> Adjacency_List;
   /*
    * Initializes the map to with size equal to number of vertices in a graph
    * Maps each vertex to a given List Object 
    */
    public GraphAdjacencyList(int number_of_vertices)
    {
        Adjacency_List = new HashMap>();
        for (int i = 1 ; i <= number_of_vertices ; i++)
        { 
            Adjacency_List.put(i, new LinkedList());
        }
    }


    /* Adds nodes in the Adjacency list for the corresponding vertex */
    public void setEdge(int source , int destination)
    {
       if (source > Adjacency_List.size() || destination > Adjacency_List.size())
       {
           System.out.println("the vertex entered in not present ");
           return;
       }
       List slist = Adjacency_List.get(source);
       slist.add(destination);
       List dlist = Adjacency_List.get(destination);
       dlist.add(source);
   }

   /* Returns the List containing the vertex joining the source vertex */
    public List getEdge(int source)
    {
        if (source > Adjacency_List.size())
        {
            System.out.println("the vertex entered is not present");
            return null;
        }
        return Adjacency_List.get(source);
    }

    /*
     * Main Function reads the number of vertices and edges in a graph.
     * then creates a Adjacency List for the graph and prints it  
     */
     public static void main(String...arg)
     {
         int source , destination;
         int number_of_edges,number_of_vertices;
         int count = 1;
         Scanner scan = new Scanner(System.in);
         try
         {
             /* Read the number of vertices and edges in graph */
             System.out.println("Enter the number of vertices and edges in graph");
             number_of_vertices = scan.nextInt();
             number_of_edges = scan.nextInt();
             GraphAdjacencyList adjacencyList = new GraphAdjacencyList(number_of_vertices);

             /* Reads the edges present in the graph */
             System.out.println("Enter the edges in graph Format : ");
             while (count <= number_of_edges)
             {
                 source = scan.nextInt();
                 destination = scan.nextInt();
                 adjacencyList.setEdge(source, destination);
                 count++;
             }

             /* Prints the adjacency List representing the graph.*/
             System.out.println ("the given Adjacency List for the graph \n");
             for (int i = 1 ; i <= number_of_vertices ; i++)
             {
                 System.out.print(i+"->");
                 List edgeList = adjacencyList.getEdge(i);
                 for (int j = 1 ; ; j++ )
                 {
                     if (j != edgeList.size())
                     {
                         System.out.print(edgeList.get(j - 1 )+"->");
                     }else
                     {
                         System.out.print(edgeList.get(j - 1 ));
                         break;
                     }  
                 }
                 System.out.println();
              }
          } 
          catch(InputMismatchException inputMismatch)
          {
              System.out.println("Error in Input Format. \nFormat : ");
          }
          scan.close();
     }
}


Output:

Enter the number of vertices and edges in graph
4 5
Enter the edges in graph Format :
1 2
2 3
3 4
4 1
1 3
the given Adjacency List for the graph 

1->2->4->3
2->1->3
3->2->4->1
4->3->1



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