Sum all the elements java arraylist

Two ways:

Use indexes:

double sum = 0;
for(int i = 0; i < m.size(); i++)
    sum += m.get(i);
return sum;

Use the "for each" style:

double sum = 0;
for(Double d : m)
    sum += d;
return sum;

Using Java 8 streams:

double sum = m.stream()
    .mapToDouble(a -> a)
    .sum();

System.out.println(sum);