Java groupingBy列表

Question

class Student {
    List<Integer> grades
}

我希望按成绩对学生进行分组，但如果每个学生只有一个成绩，groupingBy就可以工作。有没有别的lamba方法可以做到这一点？我期望像Map<Integer, List<Student>>那样的结果

Answer 1

如果您提供帮助，groupingBy将完成这项工作:您可能会在groupingBy的Javadoc中找到与此等效的版本。

final List<Student> students = ...;
// @formatter:off
students.stream()
        .flatMap(student -> student.grades.stream()
                                          .map(grade -> new StudentGrade(student, grade)))
        .collect(groupingBy(StudentGrade::getGrade, mapping(StudentGrade::getStudent, toList())));
// @formatter:on

现在，你得到了一个Map<Integer, List<Student>>。由您决定是否过滤重复项。

您将需要这些导入：

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.toList;

StudentGrade类很简单：

class StudentGrade {
  private final Student student;
  private final Integer   grade;
  public StudentGrade(Student student, Integer grade) {
    this.student = student;
    this.grade = grade;
  }
  public Student getStudent() {return student;}
  public Integer getGrade()   {return grade;}
}

Answer 2

 Map<Integer, List<Student>> collect = students.stream()
                                                  .flatMap(student -> student.grades.stream()
                                                                                    .map(grade -> new AbstractMap.SimpleEntry<>(grade,
                                                                                            student)))
                                                  .collect(Collectors.groupingBy(entry -> entry.getKey(),
                                                          Collectors.mapping(Entry::getValue, Collectors.toList())));

因为Java没有对元组实现，所以我使用了AbstractMap.SimpleEntry