基于子对象列表从列表创建映射

Question

我有一个包含子对象列表的类，如下所示：

class Student {
   private String name;
   private List<String> subjects;
}

我有一个学生类列表，如下所示：

Student [name = "John", subjects = ["Physics", "Math", "Chemistry"]]
Student [name = "Max",  subjects = ["Physics", "Chemistry"]]
Student [name = "Chris",  subjects = ["Chemistry"]]

Physics = [Student [name = "John", subjects = ["Physics", "Math", "Chemistry"]], 
          Student [name = "Max",  subjects = ["Physics", "Chemistry"]]],
Chemistry = [Student [name = "John", subjects = ["Physics", "Math", "Chemistry"]], 
          Student [name = "Max",  subjects = ["Physics", "Chemistry"]], 
          Student [name = "Chris",  subjects = ["Chemistry"]]] 
Math = [Student [name = "John", subjects = ["Physics", "Math", "Chemistry"]]]

我可以使用传统的foreach循环对数据进行分组，但我想为此使用stream API，但不能这样做。你能帮忙吗？

Answer 1

一种方法是将学生映射到多对(subject，student)，然后按主题分组。

List<Student> students = Arrays.asList(
        new Student("John", Arrays.asList("Physics", "Math", "Chemistry")),
        new Student("Max", Arrays.asList("Physics", "Chemistry")),
        new Student("Chris", Arrays.asList("Chemistry"))
);

Map<String, List<Student>> result = students.stream()
        .flatMap(student -> student
                .getSubjects()
                .stream()
                .map(s -> new Object[]{s, student})
        )
        .collect(
                Collectors.groupingBy(pair -> (String) pair[0], 
                Collectors.mapping(pair -> (Student) pair[1], Collectors.toList()))
        );

System.out.println(result);