java.lang.NullPointerException error in Java program to add 2 objects

Question

I have some trouble handling memory in Java

import java.util.Scanner;
class P4 {
    public static void main(String[] args) {
        Matrix x=new Matrix(2,4);
        Matrix y=new Matrix(2,4);
        Matrix a=new Matrix(2,4);
        Matrix b=new Matrix(2,4);
        Matrix c=new Matrix(2,4);
        x=a.input();
        y=b.input();
        c=x.add(y);
        c.display();
    }
}
class Matrix{
    private int x,y;
    double m[][];
    Scanner in=new Scanner(System.in);
    Matrix(int a1,int b1){
        x=a1;
        y=b1;
    }
    public Matrix input() {
        double m[][]=new double[x][y];
        System.out.println("Enter "+x*y+" elements : ");
        for(int i=0;i<x;i++){
            for(int j=0;j<y;j++){
                m[i][j]=in.nextDouble();
            }
        }
        System.out.println("You Entered : ");
        for(int i=0;i<x;i++){
            for(int j=0;j<y;j++){
                System.out.printf("%f\t",m[i][j]);
            }
            System.out.println();
        }
        return this;
    }
    public void display() {
        System.out.println("Elements in the Matrix are : ");
        for(int i=0;i<x;i++){
            for(int j=0;j<y;j++){
                System.out.printf("%f\t",m[i][j]);
            }
            System.out.println();
        }
    }
    public Matrix add(Matrix n) {
        Matrix temp=new Matrix(n.x,n.y);
        for(int i=0;i<x;i++){
            for(int j=0;j<y;j++){
                temp.m[i][j]=m[i][j]+n.m[i][j];
            }
        }
        return temp;
    }
    public Matrix sub(Matrix n) {
        Matrix temp=new Matrix(n.x,n.y);
        for(int i=0;i<x;i++){
            for(int j=0;j<y;j++){
                temp.m[i][j]=m[i][j]-n.m[i][j];
            }
        }
        return temp;
    }
}

In the above code I'm unable to to hold x and y in memory (tried to keep them in reference by using a dummy a and b), as a result I'm unable to call add() which cause the error.

BTW, my output:

Enter 8 elements : 
1 2 3 4 5 6 7 8
You Entered : 
1.000000    2.000000    3.000000    4.000000    
5.000000    6.000000    7.000000    8.000000    
Enter 8 elements : 
8 7 6 5 4 3 2 1
You Entered : 
Exception in thread "main" 8.000000 7.000000    6.000000    5.000000    
4.000000    3.000000    2.000000    1.000000    
java.lang.NullPointerException
    at Matrix.add(P4.java:53)
    at P4.main(P4.java:11)

line 11: c=x.add(y); line 53: temp.m[i][j]=m[i][j]+n.m[i][j]; — AndreDuarte, Sep 02 '14 at 20:13
See http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it — Raedwald, Sep 02 '14 at 21:46

Ted Hopp · Answer 1 · 2014-09-02T20:16:37.823

1

In the method input(), you are declaring:

double m[][]=new double[x][y];

This hides the instance variable m, which is left with its default value of null. Later, when you later try to access the elements of m, you are getting the NPE. Just change that line to:

m[][]=new double[x][y];

Also, in your add and sub methods, you are trying to access temp.m without initializing it. That would also generate an NPE and should be changed.

The best thing, I think, would be to change your constructor:

Matrix(int a1,int b1){
    x=a1;
    y=b1;
    m = new int[a1][b1];
}

and eliminate the initialization in input(). There's no reason to defer the initialization of m.

edited Sep 02 '14 at 20:16

answered Sep 02 '14 at 20:11

Ted Hopp

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+1. However, I would argue (pedantically) that `m` is not actually a class variable, but an instance variable. An instance variable becomes a class variable in Java when marked `static`. – Craig Otis Sep 02 '14 at 20:14
@CraigOtis - Right. Updated the answer accordingly. – Ted Hopp Sep 02 '14 at 20:15

score 0 · Answer 2 · answered Sep 02 '14 at 20:20

in the input()-method for matrix, you declare a new array, try this instead:

 public Matrix input() {
 //m = new double[x][y]; // do not instantiate a new m[][] here

And in the constructor, add the instatiation

Matrix(int a1, int b1) {
    x = a1;
    y = b1;
    m = new double[a1][b1];
}

java.lang.NullPointerException error in Java program to add 2 objects

2 Answers2