使用 Hashmap 创建多个数组列表 [英] Create multiple Array lists by using Hashmap

问题描述

我有一个名为 Student 的类，它有 String 属性 date.我有我所有学生的列表，现在我想创建多个 ArrayLists，它们按他们的 dates 分组.

I have class named Student and it has String attribute date. I have list of all my Students and now I want to create multiple ArrayLists which are grouped by their dates.

我想使用哈希图:

ArrayList students = getStudents();

Map map<String, ArrayList<Student>> = new HashMap<String, ArrayList<Student>);

for (Student i: students) {
 // There must be something
}

如何创建多个按学生属性的字符串值分组的学生数组列表?

How I can create multiple ArrayLists of Students which are grouped by their String value of their attribute?

推荐答案

使用 Java 8 Streams 的最简单方法:

The easiest way it to use Java 8 Streams :

Map<String, List<Student>> map =
    students.stream()
            .collect(Collectors.groupingBy(Student::getDate));

其中 getDate 是 Student 类的方法，您希望通过该方法对 Student 进行分组.

Where getDate is the method of Student class by which you wish to group the Students.

要完成 Java 8 之前代码的答案:

To complete the answer for pre-Java 8 code:

Map<String, List<Student>> map = new HashMap<>();
for (Student s : students) {
    List<Student> list = map.get(s.getDate());
    if (list == null) {
        list = new ArrayList<Student>();
        map.put (s.getDate(), list);
    }
    list.add (s);
}

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