Try to create two-dimensional arrays as objects

Question

I’m trying to create a simple program that allows me to create 2d (matrix) matrices with different objects.

My thought process was that I should be able to create objects with arguments (as stated at the beginning of the class and referred to in the constructor) and that would automatically create a matrix with the dimension m => linhas; n => colunas, passed as object constructor parameters. So, I tried to give values directly to each matrix cell.

So this is the code:

public class Matrix {

    Scanner ler = new Scanner (System.in);   
    int m;
    int n;
    int [][] arr = new int[m][n];

    Matrix(int a, int b){
        m = a;
        n = b;
    }


    public static void main(String[] args) {

      Matriz m1 = new Matrix(3,2);
      //System.out.println(m1.m +" "+ m1.n); -> Mostra corretamente os valores "3" e "2"
      m1.arr[0][0] = 1;
      m1.arr[1][1] = 1;
      m1.arr[2][0] = 1;
      m1.arr[0][1] = 1;
      m1.arr[1][0] = 1;
      m1.arr[2][1] = 1;


      //Matrix m2 = new Matrix(3,2);

    }
}

Netbeans showed no errors, but when I tried to run the program, I got the following message:

Exception in thread "main" java.lang.Arrayindexoutofboundsexception: 0 at pacote1.Matriz.extra(Matrix.java:32) at pacote1.Matriz.main(Matrix.java:43) C: Users me Appdata Local Netbeans Cache 8.2 executor-snippets run.xml:53: Java returned: 1 BUILD FAILED (total time: 0 Seconds)

When I looked for this error here in stackoverflow, I started to understand that this error means that I am trying to access an index that does not exist in the matrix, and it must mean that I did not succeed in trying to give values to’m' and 'n' as object arguments.

How to solve this?

Answer 1

The problem with your code is that you start the array before you receive the values to define its dimension, and this is a consequence of java class initialization order. Like the variables m and n are primitive of the type int and are not initialized, the default startup value set for them is 0 and end up starting the array with dimension 0x0. Passing the creation of the array to the constructor as below, it will only start the array after m and n receive the values desired for its dimension.

See the corrected code:

public class Matrix {

    Scanner ler = new Scanner (System.in);   
    int m;
    int n;
    int [][] arr;

    Matrix(int a, int b){
        m = a;
        n = b;
        arr = new int[m][n];
    }


    public static void main(String[] args) {

      Matrix m1 = new Matrix(3,2);
      //System.out.println(m1.m +" "+ m1.n); -> Mostra corretamente os valores "3" e "2"
      m1.arr[0][0] = 1;
      m1.arr[1][1] = 1;
      m1.arr[2][0] = 1;
      m1.arr[0][1] = 1;
      m1.arr[1][0] = 1;
      m1.arr[2][1] = 1;


      //Matrix m2 = new Matrix(3,2);

    }
}

See it working on ideone: https://ideone.com/yvhn16