[RESOLVED] NullPointerException frustration...

Hello,

I'm a student working on some homework and I can't get past or understand this small error. I'm only part way through so please ignore any other mistakes that you see but I'm having issues. I know which line of code is causing them but I don't understand why.

Exception in thread "main" java.lang.NullPointerException
at bonusProgram1.bonusProgram.readGraph(bonusProgram.java:87)
at bonusProgram1.bonusProgram.main(bonusProgram.java:21)

adjacencyMatrix[k].add(j); is causing the error. Any insight you can offer would be greatly appreciated.

Code:

---


package bonusProgram1;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.File;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Scanner;


public class bonusProgram {

        /**
        * @param args
        * @throws IOException
        */
        public static void main(String[] args) throws IOException {
                // TODO Auto-generated method stub
                readGraph();
                //printGraph();
                //shortestPath();
               
                       
       
        }

//////////////////////////////////////////////////////////////////////////////////////////////////////////////       
        private static void shortestPath() {
                // TODO Auto-generated method stub
               
        }

///////////////////////////////////////////////////////////////////////////////////////////////////////
        private static void printGraph() {
                // TODO Auto-generated method stub
               
        }

////////////////////////////////////////////////////////////////////////////////////////////////////
       
       
        @SuppressWarnings("unchecked")
        private static void readGraph() {
                // TODO Auto-generated method stub
                FileReader file = null;
                try {
                        file = new FileReader("graph.txt");
                } catch (FileNotFoundException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                }
                Scanner scanner = new Scanner(file);
                ArrayList<Integer> listBegin =  new ArrayList<Integer>();
                ArrayList[] adjacencyMatrix;
                adjacencyMatrix = new ArrayList[10];
                for(int i =0;i < 9 ; i++){
                        adjacencyMatrix [i] = new ArrayList<Integer>();
            }
                int scan = 0;
               
                System.out.println("AM!!  " + adjacencyMatrix);
                while (scanner.hasNextLine()) {
                        int line = scanner.nextInt();
                        listBegin.add(line);
                        scan += 1;
                        }
               
                for (int i1 = 0; i1 < listBegin.size(); i1++){
                        System.out.println("Reading line: " + listBegin.get(i1));
                        }
                System.out.println("AM!!  " + adjacencyMatrix);
               
               
                for (int i2 = 0; i2 < listBegin.size(); i2 += 2){
                        int j = (int)listBegin.get(i2);
                        int h = i2 +1;
                        int k = (int)listBegin.get(h);
                        adjacencyMatrix[j].add(k);
                        adjacencyMatrix[k].add(j);
                        }
               
               
                for (int i = 0; i < adjacencyMatrix.length; i++){
                        System.out.println("AM!! "+ i + adjacencyMatrix[i]);
                        }
               
        }
}

/**********************************************
 * The sample input
 * 0 9
 * 1 2
 * 1 3
 * 2 4
 * 2 5
 * 3 6
 * 4 5
 * 4 7
 * 4 8
 * 5 6
 * 5 9
 * 8 9
***************************************************/

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You problem is here:

Code:

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                for(int i =0;i < 9 ; i++){
                        adjacencyMatrix [i] = new ArrayList<Integer>();

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You are only creating ArrayList objects for the first 9 elements of your 10 element array.

It is good practice to do the following when filling arrays:

Code:

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                for(int i =0;i < adjacencyMatrix.length ; i++){
                        adjacencyMatrix [i] = new ArrayList<Integer>();

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If you use this approach then you don't have to worry what size the array is it just fills every element. Another advantage is if you decide the array should be a different size your code will still work without you having to change the for loop.

Thank you very much! I will take you advice.