Initialization of 2-D Arrays using Constructors in Java

Question

Matrix.java

    import java.io.*;
    class Matrix {
        private int q[][];

        public Matrix() {
            for(int i=0;i<3;i++)
            for(int j=0;j<3;j++)
                q[i][j] = Integer.parseInt(System.console().readLine());    
        }


        public Matrix( int a , int b ) {    
            int mat[][] = new int [a][b];
            for(int i=0; i<mat.length; i++) {
                for(int j=0;j<mat[i].length;j++)
                    q[i][j] = Integer.parseInt(System.console().readLine());
            }
        }



         public void show() {
            for(int i=0; i<q.length; i++) {
                for(int j=0;j<q[i].length;j++)
                    System.out.println(q[i][j]+" ");
            }   
        }
    }

UseMatrix.java

class UseMatrix {
    public static void main(String args[]) {

        Matrix m1 = new Matrix();  
        System.out.println("First Matrtix ");
        m1.show();

        Matrix m2 = new Matrix(5,4);
        System.out.println("Second Matrtix "); 
        m2.show();


    }
}

This programs shows NullPointerException error at runtime

Confused why this isn't working could use a little help, I want to the create a 2D Array of Size 3*3 through Default Constructors.

Then I want to create a Array of size 5*4 using parameterized constructors.

Answer 1

There are a number of problems:

• First: private int q[][]; // this holds a null value, never assigned on the parameterless constructor. A possible solution to this would be to add in your constructor: q[][] = new int[a][b]; and q[i] = new int[b];// each time you enter the loop. See the code below for clarity.

• Second in public Matrix() { you try to access the array contents without checking its length (for(int i=0;i<3;i++) goes from 0 to 3, regarles of the actual array size). The array is empty at the beggining so this causes another NullPointerException here q[i][j] = Integer.parseInt... (I will solve this reusing the other constructor because they want to achieve the same result)

• Third there is problems reading from console(), so I changed that too. (And added a line where you ask the user for a number)

• And the last change I made was a small tweak to the show method.

Result:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class App {
    public static void main(String[] args) {

        Matrix m1 = new Matrix();
        System.out.println("First Matrtix ");
        m1.show();

        Matrix m2 = new Matrix(5,4);
        System.out.println("Second Matrtix ");
        m2.show();
    }


}

class Matrix {
    private int q[][];
    private BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    public Matrix() {
        this(3, 3);
    }


    public Matrix(int a, int b ) {
        int value;
        System.out.println("Input a number to fill the new 3x3 matrix");
        try {
            value = Integer.parseInt(br.readLine());
        } catch (IOException e) {
            throw new RuntimeException("There was a problem reading the number from console", e);
        }
        q = new int[a][b];
        for(int i=0;i<a;i++) {
            q[i] = new int[b];
            for(int j=0;j<b;j++) {
                q[i][j] = value;
            }
        }
    }



    public void show() {
        for(int i=0; i<q.length; i++) {
            for(int j=0;j<q[i].length;j++)
                System.out.print(q[i][j]+" ");
            System.out.print("\n");
        }
    }
}