List<Vo> list = new ArrayList<>();
for (int i = 0; i < 4; i++) {
    Vo vo = new Vo();
    vo.setName("21");
    vo.setGrade(new Double(2 + i));
    list.add(vo);
}
double sum = list.stream().mapToDouble(Vo -> {
    Double grade = Vo.getGrade();
    return grade;
}).sum();
System.out.println(sum);