if (mark >= 0 && mark <= 29) {
catOne++;
System.out.println("");
//variable catOne might not be initialized
if (mark >= 0 && mark <= 29) {
catOne++;
System.out.println("");
//variable catOne might not be initialized
It means, you have to initialize catOne, e.g. with catOne = 0;
before reading it (witch of course the ++
operator does).