$($Optional$)$ Consider the following Test.java file.

$//$ inside main

System.out.println $($foo$($Hello$))$; $//$line $6$

$//$ more line of codes

$.....$

$.....$

public static int foo $($String s$)//$line $9$

$\{$

if $($s$.$length$()==0)$

return $0$;

else

return $(1+$ foo $($s$.$substring$(0,$ s$.$length$()-2)))$; $//$line $15$

$\}$

The code compiles, but when it runs, you get the following output:

Exception in thread $$main$$ java.lang.StringIndexOutOfBoundException: begin $0,$ end $-1,$ length $1$

at java.base$/$java$.$lang.String.checkBoundsBeginEnd$($Unknown source$)$

at java.base$/$java$.$lang.String.subString$($Unknown source$)$

at Test.foo$($Test$.$java:$15)$

at test.foo$($Test$.$java:$15)$

at Test.foo$($Test$.$java:$15)$

at test.main$($Test$.$java:$6)$

Explain what the problem is and how it happens.