Adjacency List in Java


Hello people..! This is a special extension for my discussion on Graph Theory Basics. Here, I give you the code for implementing the Adjacency List in Java. Some of the features of this code are –

  • The Adjacency List is an array of LinkedLists, where each element is a Pair<Integer, Integer>, from javafx.util.Pair. This pair stores two values, the destination vertex, (V2 in an edge V1 → V2) and the weight of the edge.
  • addEdge() – Appends an edge startVertexendVertex with a certain weight. This is an O(1) Insertion.
  • getNumberOfVertices() – Returns the number of vertices in the graph. This value does not change once the graph is created.
  • getNumberOfEdgesFromVertex() – Returns the number of edges coming out from a vertex. Basically, it returns the length of the linked list associated with the vertex.
  • getEdgesFromVertex() – Returns a LinkedList object of edges that come out of this vertex. Changes made to the object returned does not affect the encapsulated adjacency list.
  • printAdjacencyList() – Prints the adjacency list top-to-down.
  • removeEdge() – Removes an edge coming out of startVertex. It expects an object of Pair<Integer, Integer> which corresponds to the edge to be removed. Returns a boolean value regarding whether there was any change made to the Collection.
/*
 * Adjacency List in Java
 * using Collections API
 *
 * Authored by,
 * Vamsi Sangam.
 */

import java.util.LinkedList;
import java.util.Scanner;
import javafx.util.Pair;

public class AdjacencyList {
    private final LinkedList< Pair<Integer, Integer> >[] adjacencyList;
    
    // Constructor
    public AdjacencyList(int vertices) {
        adjacencyList = (LinkedList< Pair<Integer, Integer> >[]) new LinkedList[vertices];
        
        for (int i = 0; i < adjacencyList.length; ++i) {
            adjacencyList[i] = new LinkedList<>();
        }
    }
    
    // Appends a new Edge to the linked list
    public void addEdge(int startVertex, int endVertex, int weight) {
        adjacencyList[startVertex].add(new Pair<>(endVertex, weight));
    }
    
    // Returns number of vertices
    // Does not change for an object
    public int getNumberOfVertices() {
        return adjacencyList.length;
    }
    
    // Returns number of outward edges from a vertex
    public int getNumberOfEdgesFromVertex(int startVertex) {
        return adjacencyList[startVertex].size();
    }
    
    // Returns a copy of the Linked List of outward edges from a vertex
    public LinkedList< Pair<Integer, Integer> > getEdgesFromVertex(int startVertex) {
        LinkedList< Pair<Integer, Integer> > edgeList
                            = (LinkedList< Pair<Integer, Integer> >) new LinkedList();
        
        return edgeList;
    }
    
    // Prints the Adjaency List
    public void printAdjacencyList() {
        int i = 0;
        
        for (LinkedList< Pair<Integer, Integer> > list : adjacencyList) {
            System.out.print("adjacencyList[" + i + "] -> ");
            
            for (Pair<Integer, Integer> edge : list) {
                System.out.print(edge.getKey() + "(" + edge.getValue() + ")");
            }
            
            ++i;
            System.out.println();
        }
    }
    
    // Removes an edge and returns true if there
    // was any change in the collection, else false
    public boolean removeEdge(int startVertex, Pair<Integer, Integer> edge) {
        return adjacencyList[startVertex].remove(edge);
    }
}

class testGraph
{
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        
        int vertices = s.nextInt();
        int edges = s.nextInt();
        int u, v, weight;
        
        AdjacencyList adjacencyList = new AdjacencyList(vertices);
        
        int i = 0;
        
        while (i < edges) {
            u = s.nextInt() - 1;
            v = s.nextInt() - 1;
            weight= s.nextInt();
            
            adjacencyList.addEdge(u, v, weight);
            ++i;
        }
        
        adjacencyList.printAdjacencyList();
        adjacencyList.removeEdge(0, new Pair<>(1, 1));
        adjacencyList.printAdjacencyList();
    }
}

Feel free to comment if you have any doubts..! Commenting is super easy if you are a Facebook, Twitter or a Google+ user…! So, don’t hesitate..! 😉 … Keep practising..! Happy Coding..! 😀

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