Graph.java

// Java program to implement Graph
// with the help of Generics

import java.util.*;

class Graph<T> {

    // We use Hashmap to store the edges in the graph
    private Map<T, List<T> > map = new HashMap<>();

    // This function adds a new vertex to the graph
    public void addVertex(T s)
    {
        map.put(s, new LinkedList<T>());
    }

    // This function adds the edge
    // between source to destination
    public void addEdge(T source, T destination,
                        boolean bidirectional)
    {

        if (!map.containsKey(source))
            addVertex(source);

        if (!map.containsKey(destination))
            addVertex(destination);

        map.get(source).add(destination);
        if (bidirectional == true) {
            map.get(destination).add(source);
        }
    }

    // This function gives the count of vertices
    public void getVertexCount()
    {
        System.out.println("The graph has "
                           + map.keySet().size()
                           + " vertex");
    }

    // This function gives the count of edges
    public void getEdgesCount(boolean bidirection)
    {
        int count = 0;
        for (T v : map.keySet()) {
            count += map.get(v).size();
        }
        if (bidirection == true) {
            count = count / 2;
        }
        System.out.println("The graph has " + count
                           + " edges.");
    }

    // This function gives whether
    // a vertex is present or not.
    public void hasVertex(T s)
    {
        if (map.containsKey(s)) {
            System.out.println("The graph contains " + s
                               + " as a vertex.");
        }
        else {
            System.out.println("The graph does not contain "
                               + s + " as a vertex.");
        }
    }

    // This function gives whether an edge is present or
    // not.
    public void hasEdge(T s, T d)
    {
        if (map.get(s).contains(d)) {
            System.out.println(
                "The graph has an edge between " + s
                + " and " + d + ".");
        }
        else {
            System.out.println(
                "The graph has no edge between " + s
                + " and " + d + ".");
        }
    }
  
      public void neighbours(T s)
    {
        if(!map.containsKey(s)) 
        return ;
        System.out.println("The neighbours of "+s+" are");
        for(T w:map.get(s))
         System.out.print(w+",");
    }

    // Prints the adjancency list of each vertex.
    @Override public String toString()
    {
        StringBuilder builder = new StringBuilder();

        for (T v : map.keySet()) {
            builder.append(v.toString() + ": ");
            for (T w : map.get(v)) {
                builder.append(w.toString() + " ");
            }
            builder.append("\n");
        }

        return (builder.toString());
    }
}

// Driver Code
public class Main {

    public static void main(String args[])
    {

        // Object of graph is created.
        Graph<Integer> g = new Graph<Integer>();

        // edges are added.
        // Since the graph is bidirectional,
        // so boolean bidirectional is passed as true.
        g.addEdge(0, 1, true);
        g.addEdge(0, 4, true);
        g.addEdge(1, 2, true);
        g.addEdge(1, 3, true);
        g.addEdge(1, 4, true);
        g.addEdge(2, 3, true);
        g.addEdge(3, 4, true);

        // Printing the graph
        System.out.println("Graph:\n" + g.toString());

        // Gives the no of vertices in the graph.
        g.getVertexCount();

        // Gives the no of edges in the graph.
        g.getEdgesCount(true);

        // Tells whether the edge is present or not.
        g.hasEdge(3, 4);

        // Tells whether vertex is present or not
        g.hasVertex(5);
               g.neighbours(1);

    }
}