/*
output
6
this number always relevent to index number here a[6]
why ???
so far i know that ...
getMessage():
Returns the error message
string of this throwable object.
Returns:
the error message string of this Throwable object if it was created with an error
message string; or null if it was created with no error message.
.. but still don't understand why ??? it shows that index 6 (in that case )
*/