how to debug the java action

Hello Experts, i am creating a java action in which i am generating random number using Math.random() , which is returning the double value after it i convert it into String now i am splitting this string into substring using dot(.) and storing into an string array like below lets = String StrArr = Str.split('.'); then i am returning its zero index like below return StrArr[0]; but i am getting exception bounding error of array index i want to debug java action that split is working or not properly how can i debug java action