how to find the average in an arraylist

private double calculateAverage(List <Integer> marks) {
    return marks.stream()
                .mapToDouble(d -> d)
                .average()
                .orElse(0.0)
}

for(int i = 0; i<sum.size(); i++)
    total = total+sum.get(i);
avg = total / sum.size();
System.out.println("The Average IS:" + avg);