Exception in thread "main" java.lang.NullPointerException

[tuocuggino]

hi guys, i started studying java recently and today i'm stucked on this error.
can anyone help me?

import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
import java.lang.Integer;
 
public class Matrix {
    public static int[][] matNodi() throws NumberFormatException, IOException
    {
    	String file = "/home/marco/file.txt";
    	int matNodi[][] = null;
        FileInputStream inputStream = new FileInputStream(file);
        DataInputStream in = new DataInputStream(inputStream);
        BufferedReader bf = new BufferedReader(new InputStreamReader(in));
 
        int lineCount = 0;
        StringTokenizer str;
    	String line = "";
        while ((line = bf.readLine()) != null) 
        {
    		str = new StringTokenizer(bf.readLine());
        	for ( int j = 0 ; j < str.countTokens(); j++ ){
        		matNodi[lineCount][j]= Integer.parseInt(str.nextToken());	
        	}
 
            lineCount++;
        }
        bf.close();
        return matNodi;
	}
 
        public static void main(String[] args) throws NumberFormatException, IOException{
		int [][] m = matNodi();
		System.out.print(m[1][1]);
	}
}

Exception in thread "main" java.lang.NullPointerException
	at Matrix.matNodi(Matrix.java:44)
	at Matrix.main(Matrix.java:53)

[Norm]

The error message says that there was a variable with a null on line 44 when that line was executed. Look at that line, find what variable was null and backtrack in the code to see why that variable does not have a valid value.

The code appears to read from the file in two places. The record read into the line variable is not used.

[mittal]

import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
import java.lang.Integer;

public class ExceptionDemo
{

public static int[][] matNodi() throws NumberFormatException, IOException
{
String file = "D:\\Backup\\a.txt"; //a.txt file is local file in that some test.
int matNodi[][] = null;

FileInputStream inputStream = new FileInputStream(file);
DataInputStream in = new DataInputStream(inputStream);
BufferedReader bf = new BufferedReader(new InputStreamReader(in));

int lineCount = 0;
StringTokenizer str;
String line = "";
while ((line = bf.readLine()) != null)
{
str = new StringTokenizer(line);
for ( int j = 0 ; j < str.countTokens(); j++ )
{
matNodi[lineCount][j]= Integer.parseInt(str.nextToken()); // This point null pointer exception occured.
System.out.println(matNodi[lineCount][j]); // Here I try to print matNodi but before it print occured exception.
}

lineCount++;
}
bf.close();
return matNodi;
}
public static void main(String[] args) throws NumberFormatException, IOException
{
int [][] m = matNodi();
System.out.print(m[1][1]);
}
}

In above program , in for loop at "matNodi[lineCount][j]= Integer.parseInt(str.nextToken()); line NullPointerException occuured. And at line "str = new StringTokenizer(bf.readLine());" It should be as str = new StringTokenizer(line); otherwise in str goes from second line.