Beginner problem...pease help!!urgent..

[vartikachandra]

i am trying to accept a 3 by 4 integer matrix from the user and an unable to do so.here is my code.

void accept()throws IOException
	{
		int i,j;
		InputStreamReader instream = new InputStreamReader(System.in);
		BufferedReader stdin = new BufferedReader(instream);
		System.out.println("\nEnter the coefficients and the constants for the 3 equations  ");
		for(i=0;i<3;i++)
		{
			for(j=0;j<4;j++)
			{
				System.out.println("M["+i+"]["+j+"]=");
				mat1[i][j]=Integer.parseInt(stdin.readLine());
			}
		}	
	}

Thanks in advance!please help

[sillyboy]

what is your error?

[vartikachandra]

the error when irun the program::

Enter the coefficients and the constants for the 3 equations
M[0][0]=
2
value is2
Exception in thread "main" java.lang.NullPointerException
at mypack.matrix.accept(classfile.java:21)
at mypack.Linear_Equation.main(Linear_Equation.java:8)

[stephen84s]

I cannot see at what point you have put a System.out.println() for "value is2". Also show us where and how the method "accept()" is being called and how the matrix is being initialised.

Also I guess it would be nice if you would "close()" your BufferedReader, once you are done using it in that method.
The developers did not give the BufferedReader class a "close()" method just for displaying in the javadocs.

[sillyboy]

"value is2" where does this come from?

your code above has nothing about this...

[vartikachandra]

"value is2" where does this come from?

your code above has nothing about this...

there are 2 file:
Linear_Equation.java is one
and the other si classfile.java both are in mypack.

LinearEquation.java:

package mypack;

import java.io.*;

public class Linear_Equation{
    public static void main(String args[])throws IOException{
        matrix mat1=new matrix();
        mat1.accept();
        mat1.display();

    }


}

classfile.java:

package mypack;

import java.io.*;

class matrix
{
    int mat1[][];
    void accept()throws IOException
    {
        int i,j,ch;
        InputStreamReader instream = new InputStreamReader(System.in);
        BufferedReader stdin = new BufferedReader(instream);
        System.out.println("\nEnter the coefficients and the constants for the 3 equations  ");
        for(i=0;i<3;i++)
        {
            for(j=0;j<4;j++)
            {
                System.out.println("M["+i+"]["+j+"]=");
                ch=Integer.parseInt(stdin.readLine());
                System.out.println("value is"+ch);
                mat1[i][j]=ch;
                System.out.println("value is"+mat1[i][j]);
            }
        }   
    }
    void display()
    {
        int i,j;
        System.out.println("\nThe matrix is:\n");
        for(i=0;i<3;i++)
        {
            System.out.println("\n");
            for(j=0;j<3;j++)
                System.out.println(""+ mat1[i][j]);
        }   
    }
    void echolonform()
    {

    }

}

and the error is:

Enter the coefficients and the constants for the 3 equations  
M[0][0]=
2
value is2
Exception in thread "main" java.lang.NullPointerException
    at mypack.matrix.accept(classfile.java:21)
    at mypack.Linear_Equation.main(Linear_Equation.java:8)

[stephen84s]

Edit your post, the code tags go in square brackets ([) not angled brackets, like here:-

[code=java] // Your code goes here

[/code]

[sillyboy]

should use [] for code tags...

anyway, it's hard to read but i think need to create a proper constructor for matrix. you cannot use an array like a vector.

[stephen84s]

According to me this is what is causing the problem :-

int mat1[][];

You have never initialized your matrix, with any size.
You will have to initialize your matrix as follows to be able to use it:-

int[][] mat1 = new int[3][4];

Heres a tutorial to help you on arrays.

[vartikachandra]

thanks!!this is working jus fine!!