JAVA8 集合类的常见用法

查看所有学生一共有多少个班级

Map<String, Student> studentMap= studentList.stream()
                .collect(Collectors.toMap(x -> x.getClassId(), x -> x, (x, y) -> {
                    log.debug(x + " Merge " + y);
                    return x;
                }));
 log.debug("studentMap: " + studentMap);

将学生按班级分组

 Map<String, List<Student>> studentMap= studentList.stream()
                .limit(studentList.size())
                .collect(Collectors.groupingBy(Student::getClassId));

        log.debug("studentMap: " + studentMap);

将学生取出集合中所有班级的所有学生

Set<Student> studentSet = studentList.stream().
              filter(x -> studentMap.keySet().contains(x.getClassId()))
              .collect(Collectors.toSet());

     log.debug("\nfilter studentSet .size() : " + studentSet.size());
      log.debug("\n");
        log.debug("\n studentSet : " + studentSet );