http://www.sanfoundry.com/java-program-describe-representation-graph-using-adjacency-list/

GraphAdjacencyList.java

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<Integer, List<Integer>> 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<Integer, List<Integer>>();
        for (int i = 1 ; i <= number_of_vertices ; i++)
        {
            Adjacency_List.put(i, new LinkedList<Integer>());
        }
    }


    /* 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<Integer> slist = Adjacency_List.get(source);
        slist.add(destination);
        List<Integer> dlist = Adjacency_List.get(destination);
        dlist.add(source);
    }

    /* Returns the List containing the vertex joining the source vertex */
    public List<Integer> 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 : <source index> <destination index>");
            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<Integer> 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 : <source index> <destination index>");
        }
        scan.close();
    }
}

this doesnt seem dynamic, as in you have to specify before hand the no of vertices or edges leaving very little room for flexibility

it doesn't check whether the new edge is already present or not.

it's ok....you can just put one function
public boolean hasEdge(int src, int dest)
{
return adjacencyList.get(src).contains(dest);

}

verify this function in addEdges()