Basics In Java

import java.io.*;
class arr
{
     public static void main()throws IOException
     {
         DataInputStream k=new DataInputStream(System.in);
         System.out.println("Fill The Array");
         int a[][]=new int[4][4];
         for(int i=0;i<4;i++)
         for(int j=0;j<4;j++)
         a[i][j]=Integer.parseInt(k.readLine());
         for(int i=0;i<4;i++)
         {
             for(int j=0;j<4;j++)
             System.out.print(a[i][j]+" ");
             System.out.println();
         }
         for(int i=0;i<4;i++)
         {
             int t=a[i][i];
             a[i][i]=a[i][3-i];
             a[i][3-i]=t;
         }
         System.out.println("After Arranging");
         for(int i=0;i<4;i++)
         {
             for(int j=0;j<4;j++)
             System.out.print(a[i][j]+" ");
             System.out.println();
         }
        }
    }