Lambda 按Java8分组

假设你是一名注册课程的学生

Class Student{
  ArrayList<Course> courses;
}
Class Course{
  String id;
  String name;
}

班级学生{
ArrayList课程；
}
班级课程{
字符串id；
字符串名；
}

---

如何在java 8中使用groupBy函数列出参加特定课程的学生

当您想将同一个学生划分为不同的组时，

groupingBy

收集器不适用于此处（它将每个流元素精确地放在一个组中）

创建一个可变容器（如

HashMap

）并通过

forEach

填充它会更有效：

Map<Course, List<Student>> result = new HashMap<>();
students.forEach(student -> student.courses.forEach(
        course -> result.computeIfAbsent(course, c -> new ArrayList<>()).add(student)));

看起来有点难看。“我的库”为此类场景添加了一些快捷方式：

Map<Course, List<Student>> result = StreamEx.of(students)
      .cross(s -> s.courses.stream()).invert().grouping();

Map result=streamx.of（学生）
.cross（s->s.courses.stream（））.invert（）.grouping（）；

---

这看起来好得多，但简单的解决方案看起来仍然更好（并且不需要第三方库）。

我必须承认，我使用的是@tagir valeev answer的想法。
创建一个简单的类

public class CourseStundentPair {
    private Course course;
    private Student student;

    public CourseStundentPair(Course course, Student student) {
        this.course = course;
        this.student = student;
    }
    ...
    }

这将使代码更加简洁易读。
只是出于说服力，我给学生添加了名称字段

import static java.util.stream.Collectors.*;

public class TestCourse {

    public static void main(String[] args) {
        Student student1 = new Student("1", "Student1");
        Student student2 = new Student("2", "Student2");
        Student student3 = new Student("3", "Student3");
        Student student4 = new Student("4", "Student4");
        Course course1 = new Course("c1", "English");
        Course course2 = new Course("c2", "Mathematics");
        Course course3 = new Course("c3", "Literature");
        student1.getCourses().add(course1);
        student1.getCourses().add(course2);
        student1.getCourses().add(course3);

        student2.getCourses().add(course1);
        student2.getCourses().add(course2);
        student3.getCourses().add(course1);

        List<Student> students = new ArrayList<>();
        students.add(student1);
        students.add(student2);
        students.add(student3);
        students.add(student4);

        final Map<Course, List<Student>> courseToStudents = students.stream()
            .flatMap(s -> s.getCourses().stream().map(c -> new CourseStundentPair(c,s)))
            .collect(groupingBy(CourseStundentPair::getCourse,
                                mapping(CourseStundentPair::getStudent, toList())
                    )
            );

       for (Course course :courseToStudents.keySet()){
           System.out.printf("Course %s , Students: %s \n"
                   ,course.getName()
                   ,courseToStudents.get(course).stream().map(Student::getName)
                                    .collect(joining(", ")));
       }

    }

---

请注意，在eclipse中，IDE可能会抱怨编译错误，而在Intellij中，它可以正常工作。

谢谢Tagir Valeev。这很有用。显式类型

应该是不必要的。

import static java.util.stream.Collectors.*;

public class TestCourse {

    public static void main(String[] args) {
        Student student1 = new Student("1", "Student1");
        Student student2 = new Student("2", "Student2");
        Student student3 = new Student("3", "Student3");
        Student student4 = new Student("4", "Student4");
        Course course1 = new Course("c1", "English");
        Course course2 = new Course("c2", "Mathematics");
        Course course3 = new Course("c3", "Literature");
        student1.getCourses().add(course1);
        student1.getCourses().add(course2);
        student1.getCourses().add(course3);

        student2.getCourses().add(course1);
        student2.getCourses().add(course2);
        student3.getCourses().add(course1);

        List<Student> students = new ArrayList<>();
        students.add(student1);
        students.add(student2);
        students.add(student3);
        students.add(student4);

        final Map<Course, List<Student>> courseToStudents = students.stream()
            .flatMap(s -> s.getCourses().stream().map(c -> new CourseStundentPair(c,s)))
            .collect(groupingBy(CourseStundentPair::getCourse,
                                mapping(CourseStundentPair::getStudent, toList())
                    )
            );

       for (Course course :courseToStudents.keySet()){
           System.out.printf("Course %s , Students: %s \n"
                   ,course.getName()
                   ,courseToStudents.get(course).stream().map(Student::getName)
                                    .collect(joining(", ")));
       }

    }

Course English , Students: Student1, Student2, Student3 
Course Literature , Students: Student1 
Course Mathematics , Students: Student1, Student2