In Python (my personal favorite...also check out sage and https://www.cocalc.com), here's how I would do it:

class Averager:
        l = []
        def add(self,a):
                self.l.append(a)
        def getAverage(self):
                return sum(self.l)/float(len(self.l))
        def getSum(self):
                return sum(self.l)
        def getNum(self):
                return len(self.l)
        def reset(self):
                self.l = []