将对象列表转换为Map< Long，List>>.使用流 [英] Convert a list of objects to Map<Long, List<>> using streams

问题描述

我想使用 streams 将Student对象的列表转换为 Map< Long，List<>> .

I want to convert a list of Student objects to Map<Long, List<>> using streams.

List<Student> list = new ArrayList<Student>();
list.add(new Student("1", "test 1"));
list.add(new Student("3", "test 1"));
list.add(new Student("3", "test 3"));

我希望通过以下方式获得最终结果:

I want the final outcome in the following way:

地图

键:1

值列表: Student("1"，"test 1")

键:3

值列表:学生("3"，测试1")，学生("3"，测试3")

我尝试了以下代码，但是它正在重新初始化 Student 对象.谁能帮我修复以下代码?

I tried the following code, but it is reinitializing the Student objects. Can anyone help me fix the below code?

Map<Long, List<Student>> map = list.stream()
                        .collect(Collectors.groupingBy(
                                Student::getId,
                                Collectors.mapping(Student::new, Collectors.toList())
                        ));

推荐答案

您不需要链接 mapping 收集器.默认情况下，单个参数 groupingBy 将为您提供 Map< Long，List< Student>> .

You don't need to chain the mapping collector. The single argument groupingBy will give you a Map<Long, List<Student>> by default.

Map<Long, List<Student>> map = 
    list.stream()
        .collect(Collectors.groupingBy(Student::getId));

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