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Graph Representation using Java ArrayList

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  • Difficulty Level : Medium
  • Last Updated : 16 Apr, 2019
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Prerequisite : Graph and its representations

In this article, we will be discussing Adjacency List representation of Graph using ArrayList in Java.

Following is adjacency list representation of the above graph.

Adjacency List Representation of Graph

The idea is to use ArrayList of ArrayLists.




// Java code to demonstrate Graph representation
// using ArrayList in Java
import java.util.*;
  
class Test {
  
    static void addEdge(ArrayList<ArrayList<Integer> > adj,
                        int u, int v)
    {
        adj.get(u).add(v);
        adj.get(v).add(u);
    }
  
    static void printAdjacencyList(ArrayList<ArrayList<Integer> > adj)
    {
        for (int i = 0; i < adj.size(); i++) {
            System.out.println("Adjacency list of " + i);
            for (int j = 0; j < adj.get(i).size(); j++) {
                System.out.print(adj.get(i).get(j) + " ");
            }
            System.out.println();
        }
    }
  
    public static void main(String[] args)
    {
        // Creating a graph with 5 vertices
        int V = 5;
        ArrayList<ArrayList<Integer> > adj = new ArrayList<ArrayList<Integer> >(V);
        for (int i = 0; i < V; i++)
            adj.add(new ArrayList<Integer>());
  
        // Adding edges one by one.
        addEdge(adj, 0, 1);
        addEdge(adj, 0, 4);
        addEdge(adj, 1, 2);
        addEdge(adj, 1, 3);
        addEdge(adj, 1, 4);
        addEdge(adj, 2, 3);
        addEdge(adj, 3, 4);
        printAdjacencyList(adj);
    }
}


Output:

Adjacency list of 0
1 4 
Adjacency list of 1
0 2 3 4 
Adjacency list of 2
1 3 
Adjacency list of 3
1 2 4 
Adjacency list of 4
0 1 3

Similar Implementation using a separate Class for Graph.




// Java code to demonstrate Graph representation
// using ArrayList in Java
import java.util.*;
  
class Graph {
    ArrayList<ArrayList<Integer> > adj;
    int V;
    Graph(int v)
    {
        V = v;
        adj = new ArrayList<ArrayList<Integer> >(V);
        for (int i = 0; i < V; i++)
            adj.add(new ArrayList<Integer>());
    }
  
    void addEdge(int u, int v)
    {
        adj.get(u).add(v);
        adj.get(v).add(u);
    }
  
    void printAdjacencyList()
    {
        for (int i = 0; i < adj.size(); i++) {
            System.out.println("Adjacency list of " + i);
            for (int j = 0; j < adj.get(i).size(); j++) {
                System.out.print(adj.get(i).get(j) + " ");
            }
            System.out.println();
        }
    }
}
  
class Test {
  
    public static void main(String[] args)
    {
        // Creating a graph with 5 vertices
        int V = 5;
  
        Graph g = new Graph(V);
  
        // Adding edges one by one.
        g.addEdge(0, 1);
        g.addEdge(0, 4);
        g.addEdge(1, 2);
        g.addEdge(1, 3);
        g.addEdge(1, 4);
        g.addEdge(2, 3);
        g.addEdge(3, 4);
        g.printAdjacencyList();
    }
}


Output:

Adjacency list of 0
1 4 
Adjacency list of 1
0 2 3 4 
Adjacency list of 2
1 3 
Adjacency list of 3
1 2 4 
Adjacency list of 4
0 1 3

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