Please I need help now I have only 10 minutes any one help me

Which of the following exceptions could occur when the following code is run, if the precondition is met? // precondition: a is not null public string dosomething(arraylist a, int b) { int i; string s; for (i = 0; i < = a.size(); i++) { s = (string)(a.get(i)); if (s.length() == b) { return s; } } return null; }