Why is this code generating a NullPointerException?

I want to load the initial positions of two robots in a bidimensional array, they are indicated by the letters N,S,O,E.

The following code ain't working. Why?

static Point[] robotInitialPositions(char [][]inputMatrix){

     Point [] helperArray = new Point[2];

     int aux=0;

     for (int i=0; i<(inputMatrix[0].length-1); i++)
         for (int j=0; j<(inputMatrix[0].length-1); j++)
         {
            if((inputMatrix[i][j]=='N')||(inputMatrix[i][j]=='S')||(inputMatrix[i][j]=='O')||(inputMatrix[i][j]=='E'))
            {
                    helperArray[aux++]= new Point(i,j);                 
            }

         }

     System.out.println("helper array 1: i,j " + helperArray[1].i + ", " + helperArray[1].j);
     //NullPointerException here 
     return helperArray;

 }

Full code:

package bfs_robots;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;

class Point {

    int i;
    int j;

    Point(int i, int j){
        this.i=i;
        this.j=j;
    }

}

public class Main {




 static char turnCounter (char orientation){

     if(orientation=='N')
         return 'O';
     if(orientation=='O')
         return 'S';
     if (orientation=='S')
         return 'E';
     else
         return 'N';

 }

 static char turnClock(char orientation){

      if(orientation=='N')
         return 'E';
     if(orientation=='E')
         return 'S';
     if (orientation=='S')
         return 'O';
     else
         return 'N';

 }

 static Point[] robotInitialPositions(char [][]inputMatrix){

     Point [] helperArray = new Point[2];

     int aux=0;

     for (int i=0; i<(inputMatrix[0].length-1); i++)
         for (int j=0; j<(inputMatrix[0].length-1); j++)
         {
            if((inputMatrix[i][j]=='N')||(inputMatrix[i][j]=='S')||(inputMatrix[i][j]=='O')||(inputMatrix[i][j]=='E'))
            {
                    helperArray[aux++]= new Point(i,j);                 
            }

         }

     System.out.println("helper array 1: i,j " + helperArray[1].i + ", " + helperArray[1].j);

     return helperArray;

 }



static void bfs_find_solution (char[][] inputMatrix){


    int countOfMovements=0;
    //  each turn and displacement adds one

    //  when moved N,S,D and O must be replaced with .
    // * indicates wall, invalid movement

    Point robotInitial[] = robotInitialPositions(inputMatrix);





}









    public static void main(String[] args) throws IOException {


        BufferedReader br = new BufferedReader(new FileReader(new File("input.txt")));

        char [][] inputMatrix;

        String line;
        char [] lineAsCharArray;
        int matrixSize;

        while(true){

            line = br.readLine();
            matrixSize=Integer.parseInt(line);

            inputMatrix = new char [matrixSize][matrixSize];

            if (matrixSize==0){  // end outer looping
                break;
            }

            else { //begin inner looping

                for (int i=0; i<matrixSize; i++){

                    line = br.readLine();
                    inputMatrix[i] =line.toCharArray();

           开发者_运维问答     }

                bfs_find_solution(inputMatrix);
            }


        }

    }

}

input.txt (0 indicates the end of file)

5
D....
N...S
.....
*...*
....D
5
.....
S..S.
.....
.....
D..D.
3
SN.
***
.DD
0

     for (int i=0; i<(inputMatrix[0].length-1); i++)         
         for (int j=0; j<(inputMatrix[0].length-1); j++)

Seems wrong. The first line should be inputMatrix.length-1

Also the "<" should be "<=" I think. Or keep the "<" and don't have the "length-1", instead just "length"

One reason it's helpful to provide line numbers of errors that the system provides to you is that it makes you look at them as well. You'd notice that your error is from this line

System.out.println("helper array 1: i,j " + helperArray[1].i + ", " + helperArray[1].j);

It happens because helperArray is an array of pointers (of length 2). It initially is all null. It's supposed to get initialized somewhere in that nested loop. So I put in a print statement to see which elements get initialized and see

Setting point 0
Exception in thread "main" java.lang.NullPointerException
    at Main.robotInitialPositions(Main.java:66)
    at Main.bfs_find_solution(Main.java:83)
    at Main.main(Main.java:130)

that only point 0 gets set. But you're trying to print point 1, which is still null. So that's why you're getting your error, I'm not sure what your code is supposed to do so I'll let you figure out what SHOULD happen.