【java8】對list進行分組求和

場景

有一個student類，name是姓名，score是分數，現在要統計每位學生的總成績

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>