Make This Java code more efficient

Question

This Code is work but i got error at last after the execution of my code i.e Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1 at AddNum.main(AddNum.java:24), can any one fix this issue or make it more efficient to use.

import java.util.Scanner;
public class AddNum {
public static void main(String args[])
{
    int num1,num2,sum,i=0;
    int[] arr =new int[5];
    System.out.println("Enter Any two number to Add and See magic");
    Scanner scan = new Scanner(System.in);
    num1 = scan.nextInt();
    num2 = scan.nextInt();
    sum = num1 + num2;
    System.out.println("The Sum of "+num1+" and "+num2+" is = "+sum );
    do
    {
        arr[i++]=sum%10;
        sum/=10;
    }while(sum>0);
    for(int p=i;p>=0;p--)
    {
        mag(arr[p-1]);
    }
}
public static void mag(int sum)
{
    switch(sum){
        case 0:
            System.out.println("    00000");
            System.out.println("   00   00");
            System.out.println("  000   000");
            System.out.println("  000   000");
            System.out.println("  000   000");
            System.out.println("  000   000");
            System.out.println("  000   000");
            System.out.println("   00   00");
            System.out.println("    00000");
            break;

        case 1:
        System.out.println("      11");
        System.out.println("     111");
        System.out.println("   1 111");
        System.out.println("     111");
        System.out.println("     111");
        System.out.println("     111");
        System.out.println("     111");
        System.out.println("     111");
        System.out.println("    11111");
        break;
        case 2:
            System.out.println("    2222");
            System.out.println("  22    22");
            System.out.println("  2     222");
            System.out.println("        22");
            System.out.println("       22");
            System.out.println("     22");
            System.out.println("   222    2");
            System.out.println("  222    22");
            System.out.println("  222222222");
            break;
        case 3:
            System.out.println("    3333");
            System.out.println("  33   333");
            System.out.println("  3     333");
            System.out.println("       33");
            System.out.println("     333");
            System.out.println("       33");
            System.out.println("  3     333");
            System.out.println("  33   333");
            System.out.println("    3333");
            break;
        case 4:
            System.out.println("        44");
            System.out.println("      4444");
            System.out.println("     4 444");
            System.out.println("    4  444  ");
            System.out.println("   4   444");
            System.out.println("  4444444444");
            System.out.println("       444  ");
            System.out.println("       444");
            System.out.println("      44444");
            break;
        case 5:
            System.out.println("  5555555");
            System.out.println("  555    5");
            System.out.println("  555");
            System.out.println("  555");
            System.out.println("  555555");
            System.out.println("       555");
            System.out.println("  5     555");
            System.out.println("  55   555");
            System.out.println("    5555");
            break;
        case 6:
            System.out.println("    66666");
            System.out.println("   66    66 ");
            System.out.println("  666     6");
            System.out.println("  666");
            System.out.println("  6666666");
            System.out.println("  666   666");
            System.out.println("  666    666");
            System.out.println("   66   666");
            System.out.println("    66666");
            break;
        case 7:
            System.out.println("   777777777");
            System.out.println("  77     777");
            System.out.println("  7     777");
            System.out.println("       77");
            System.out.println("      77");
            System.out.println("     77");
            System.out.println("    777");
            System.out.println("   777");
            System.out.println("   777");
            break;
        case 8:
            System.out.println("    888888");
            System.out.println("   888  888");
            System.out.println("   888  888");
            System.out.println("    888888");
            System.out.println("   888  888");
            System.out.println("  888    888");
            System.out.println("  888    888");
            System.out.println("   888  888");
            System.out.println("     8888");
            break;
        case 9:
            System.out.println("    99999");
            System.out.println("   999 999");
            System.out.println("  999   999");
            System.out.println("  999   999");
            System.out.println("    9999999");
            System.out.println("        999");
            System.out.println("  9     999");
            System.out.println("  99   999");
            System.out.println("    9999");
            break;





    }
} }

Answer 1

You are getting ArrayIndexOutOfBounds because you are accessing the element of the array which does not exist in your array, So what you have to do is get the user input and calculate the sum for it and convert it into an array so you can do this by replacing this piece of code:-

 do
    {
        arr[i++]=sum%10;
        sum/=10;
    }while(sum>0);
    for(int p=i;p>=0;p--)
    {
        mag(arr[p-1]);
    }

By the following code:-

 int[] arr = new int[String.valueOf(sum).length()];
    for(int p=0;p<arr.length;p++)
    {
        mag(arr[p]);
    }

So the final program would look like this:-