Java Graph Implementation: nullpointerexception

Question

I am trying to implement a graph in Java using an arrayList of arrayList .

I keep getting a NullPointerException whenever the addEdge function is called. I cannot seem to figure out why.

Here is my code:

import java.util.ArrayList;

public class Graph {

    private static ArrayList<ArrayList<Integer>> adjList;

    public Graph(int vertices){
        ArrayList<ArrayList<Integer>> adjList = new ArrayList<ArrayList<Integer>>();
        for(int i = 0; i < vertices; i++){
            adjList.add(new ArrayList<Integer>());
        }
    }

    public void addEdge(int source, int destination){
        adjList.get(source).add(destination);
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Graph g = new Graph(4);
        g.addEdge(0, 1);
        g.addEdge(0, 2);
        g.addEdge(1, 2);
        g.addEdge(2, 0);
        g.addEdge(2, 3);
        g.addEdge(3, 3);

        System.out.println("Neighbors of vertex 0: " + adjList.get(0));
        System.out.println("Neighbors of vertex 2: " + adjList.get(2));
    }
}

Please kindly advise.

Answer 1

In your Graph constructor, you're not initializing the static member adjList , but rather are defining a local one with the same name. Also, there's no need adjList to be static , because it will be shared among all instances of Graph .

Adjust it to:

private ArrayList<ArrayList<Integer>> adjList;

public Graph(int vertices){
    adjList = new ArrayList<ArrayList<Integer>>();
    ...
}

Answer 2

You should remove static in adjList field declaration.

This modifier makes adjList a static instance which refers to null . And in the constructor you instantiate value for another adjList that is local for your constructor (and is subject to be gathered by GC after the constructor is called). It is two completely different variables with the same name

Answer 3

change your constructor so that you are not declaring a local adjList variable

public Graph(int vertices){
    adjList = new ArrayList<ArrayList<Integer>>();
    for(int i = 0; i < vertices; i++){
        adjList.add(new ArrayList<Integer>());
    }

}

Also make this adjList variable non static since you want the adjList to be unique to each graph and not shared across all of them

private ArrayList<ArrayList<Integer>> adjList;

Answer 4

In the Graph constructor, you declare an adjList variable, which is in conflict with your static class. You should replace

public Graph(int vertices){
        ArrayList<ArrayList<Integer>> adjList = new ArrayList<ArrayList<Integer>>();
        for(int i = 0; i < vertices; i++){
            adjList.add(new ArrayList<Integer>());
        }

    }

by

public Graph(int vertices){
        adjList = new ArrayList<ArrayList<Integer>>();
        for(int i = 0; i < vertices; i++){
            adjList.add(new ArrayList<Integer>());
        }

    }