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The problem is that sometimes I, like others, don't know we're doing it wrong.

I didn't realize I could get kicked out for giving advice, especially advice that seems to make sense to me at the...

String rollNumber = console.nextLine();

String number = rollNumber.substring(3);

Where do you want them stored at in the array?

Also, I don't know what:

System.out.print("Enter the size of the array s: ");
size = scan.nextInt();
System.out.println("size is :" + size);
String[][] arr = new...

It should be

System.out.println(rollnumber.split("IAD")[0]);

I see now that the 0 would be index 0 of the array the split method is creating.

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1

Doesn't seem right to me, with the [] around the 1.

If anything, it'd be split("IAD", 1);

Actually, it could be

System.out.println(Arrays.toString(rollnumber.split("IAD", 1)));

Also, your for loop will change letter found the first time it finds the letter and it'll still be true for the next iteration of your for loop.

System.out.println(rollnumber.split("IAD")[1]);

You're printing an array?

Also, I'm positive that's invalid syntax.

It's

public String[] split(String regularExpression)
{