reading from text file to 2d array

Could someone please tell me why I get a NullPointerException?

System.out.print("Enter the name of the file: ");
String fileName = readInput();

BufferedReader br = new BufferedReader(new FileReader(fileName + ".txt"));
String line;
int rowIndex = 0;
String matrix [][] = null;

while ((line = br.readLine()) != null) {
StringTokenizer st = new StringTokenizer(line);
int colIndex = 0;
while (st.hasMoreTokens()) {
matrix [rowIndex][colIndex++] = st.nextToken().trim();

}
}
rowIndex = rowIndex + 1;
br.close();
}

You never actually assign matrix to a new array object. The line

Code:

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String matrix [][] = null;

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leaves matrix as null, so when you try to load a value into it with

Code:

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matrix [rowIndex][colIndex++] = st.nextToken().trim();

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it's trying to assign something to null so it throws a NullPointerException.

You'll have to set the size of matrix at some point, ie matrix = new String[10][10]

I can't seem to get this right! :(
What I want to do is - Read in an adjacency matrix...
here is some example input:
2 ---> this is the order of the matrix
1 ---> this is the source node
3 4
5 6
the rest of it is the matrix

I want to put this matrix into a 2d Array, the problem is this line...String matrix [][] = new String [3][3];
I do not want it to be [3][3] I want it to be [order][order]
I've tried a few things but it doesn't work!
Any help would be really appreciated :) Thanks

Code:

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import java.io.*;
import java.util.StringTokenizer;


public class Asgn2backup {
//        public static String [][] matrix;
        public static void main(String[] args) throws IOException {
                System.out.print("Enter the name of the file: ");
                String fileName = readInput();
               
                BufferedReader br = new BufferedReader(new FileReader(fileName + ".txt"));
                String line;
                int order = 0;

                String matrix [][] = new String [3][3];

                int rowIndex = 0;
                int counter = 0;
                while ((line = br.readLine()) != null) {
                        counter++;
                        if(counter == 1){
                                order = Integer.parseInt(line);
                                System.out.println("order: " + order);
                        }
                       
                        if(counter == 2){
                       
                                String source = line;
                                System.out.println("source " + source);
                        }
                       
                        if(counter !=2 && counter !=1){
                                StringTokenizer theLine = new StringTokenizer(line);
                                int colIndex = 0;
                                while(theLine.hasMoreTokens()){
                                        String st = theLine.nextToken();//.trim();
                                        matrix [rowIndex][colIndex] = st;
                                        colIndex = colIndex + 1;
                                }
                                rowIndex = rowIndex + 1;
                        }       

                }
               
                for(int x = 0; x<matrix.length; x++){
                        for(int p = 0; p<matrix.length; p++){
                                System.out.print(matrix[x][p] + " ");
                        }
                }
               

                br.close();
        }
       
        private static String readInput() {
                try {
                        BufferedReader in = new BufferedReader(
                                new InputStreamReader(System.in));
                        return in.readLine();
                }
                catch (IOException e) {}
                return "";
        }       
       
}

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the following code will read a string from a file, convert it to an integer and create a single array based on it. you can modify it to achieve your goal:

Code:

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BufferedReader br = new BufferedReader(new FileReader("c:\my.txt"));
String line = br.readLine();

if(line == null){
  System.out.print("File is empty\n");
  System.exit(-1);
}

int number = Integer.parseInt(line);

String[] s = new String[number];

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Thanks cjard, I did try that before but something else was wrong which I fixed :)
I have another problem now, that code works fine for one adjacency matrix for example:

2 --- order
1 ---- source
0 4
0 0

but what if the input was:
2
1
0 4
0 0
3
2
0 1 0
0 0 2
3 0 0

I'm trying to figure out how to store one matrix (which I have done) and then do the same thing for the next one.

Code:

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import java.io.*;
import java.util.StringTokenizer;


public class Asgn2backup {
        public static String [][] matrix;
        public static void main(String[] args) throws IOException {
                System.out.print("Enter the name of the file: ");
                String fileName = readInput();
               
                BufferedReader br = new BufferedReader(new FileReader(fileName + ".txt"));
                String line;
                int order = 0;

                int rowIndex = 0;
                int counter = 0;
                while ((line = br.readLine()) != null) {
                        counter++;
                        if(counter == 1){
                                order = Integer.parseInt(line);
                                matrix = new String [order][order];
                                System.out.println("order: " + order);
                        }
                       
                        if(counter == 2){
                       
                                String source = line;
                                System.out.println("source: " + source);
                        }
                       
                        if(counter !=2 && counter !=1){
                                StringTokenizer theLine = new StringTokenizer(line);
                                int colIndex = 0;
                                while(theLine.hasMoreTokens()){
                                        String st = theLine.nextToken();//.trim();
                                        matrix [rowIndex][colIndex] = st;
                                        colIndex = colIndex + 1;
                                }
                                rowIndex = rowIndex + 1;
                        }       

                }
               
                for(int x = 0; x<matrix.length; x++){
                        for(int p = 0; p<matrix.length; p++){
                                System.out.print(matrix[x][p] + " ");
                        }
                }
               

                br.close();
        }
       
        private static String readInput() {
                try {
                        BufferedReader in = new BufferedReader(
                                new InputStreamReader(System.in));
                        return in.readLine();
                }
                catch (IOException e) {}
                return "";
        }       
       
}

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