reading from text file to 2d array

[cherbear]

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();
}

[reinkesm]

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

String matrix [][] = null;

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

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

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]

[cherbear]

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

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 "";
	}	
	
}

[cjard]

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:

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];

[cherbear]

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.

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 "";
	}	
	
}