Program to Average an Indefinite List of Integers

Problem: "Write a program called AverageList that reads in a list of integers representing exam scores, and then prints out the average. Because some unprepared student might actually get a score of 0, your program should use -1 as the sentinel to mark the end of the input." (Roberts Ch 4, Exercise 5.)

Program in Eclipse Editor (click to enlarge):

What the output looks like:

What made it tough: One of the earlier problems in this set asked "Write a program that finds the average of a pre-defined list of integers". That was easier, because if the list took in N inputs from the user, you'd know to divide the sum of your integers by N. Another problem asked, "Write a program that finds the sum of an indefinite list of integers." That was also easier, because you could keep adding to your sum until the user entered the sentinel. This one took me a while to figure out how you'd know what to divide your sum by, but it eventually cemented in my head the ways you can use increment operators like i++.

Time to completion: 3 hrs

Lingering Questions: Although my program says that it can average non-negative integers, you really can average any integer except the one used as the sentinel. Ben wants to know why you can't just use a letter as your sentinel, so that you could average any possible integer. I tried setting the sentinel constant as "A." Eclipse didn't like that. Is there a way to design this program so that you could use a letter as your sentinel?