【java8】对list进行分组求和

场景

有一个student类，name是姓名，score是分数，现在要统计每位学生的总成绩

方法一

studentList.stream().collect(Collectors.toMap(Student::getName, Student::getScore, Integer::sum));

方法二

List<student> studentList = new ArrayList<>();
studentList.stream()
       .collect(Collectors.groupingBy(Student::getName, Collectors.collectingAndThen(
           Collectors.summarizingDouble(Student::getScore), DoubleSummaryStatistics::getSum)));

方法三

List<student> studentList = new ArrayList<>();
studentList.stream()
       .collect(Collectors.groupingBy(Student::getName, Collectors.collectingAndThen(
            Collectors.mapping(Student::getScore, Collectors.reducing(Integer::sum)),
            Optional::get)));
</student></student>