java Why is this program giving a NullPointerException?

Question

This is my program to reverse all the elements of a matrix individually. (ie 23 goes to 32, 1427 goes to 7241, etc). It is giving 'java.lang.NullPointerException' in the fillArray() method, right after displaying the print statement. I tried using InputStreamReader throughout the code and it still did not work. Please help. Thanks!

import java.io.*;
import java.util.*;
public class MatRev
{int m,n,arr[][];
    MatRev(int mm, int nn)
    {m=mm;
     n=nn;
     int arr[][]=new int[m][n];
    }
    
    
    public void fillArray() 
    {Scanner sc=new Scanner(System.in);
        System.out.println("Please enter elements in array");
     
     for(int i=0;i<m;i++)
     {for(int j=0;j<n;j++)
         {arr[i][j]=sc.nextInt();
            }
        }
    }
public int reverse(int x)
{int r=0,g=0;
    do 
    {g=x%10;
        x=x/10;
        r=r*10+g;
    }while(x!=0);
    return r;
}
    
void revMat(MatRev p)
{for(int i=0;i<m;i++)
    {for(int j=0;j<n;j++)
        {this.arr[i][j]=reverse(p.arr[i][j]);
        }
}
}

void show()
{System.out.println("Display in matrix format");
    for(int i=0;i<m;i++)
    {for(int j=0;j<n;j++)
      {System.out.print(arr[i][j]+" ");
        }
        System.out.println();
    }
}

public void main()throws IOException
{ InputStreamReader read=new InputStreamReader(System.in);
     BufferedReader buf=new BufferedReader(read);
     System.out.println("Enter no of rows followed by no of columns");
    int a=Integer.parseInt(buf.readLine());
    int b=Integer.parseInt(buf.readLine());
    MatRev ob1=new MatRev(a,b);
    ob1.fillArray();
    System.out.println("Original matrix");
    ob1.show();
    MatRev ob2=new MatRev(a,b);
    ob2.revMat(ob1);
    System.out.println("Reverse matrix");
    ob2.show();
}}