如何将List<P>中的元素分组到Map<K、List<V>>中，同时保持顺序？

Map<String, List<PlaceSummary>> placesGroupedByPlaceId =
            places.stream()
                  .collect(Collectors.groupingBy(
                          PlaceSummary::getPlaceId,
                          LinkedHashMap::new,
                          Collectors.mapping(PlaceSummary::getPlaceId, toList())
                  ));

    Map<String, List<PlaceSummary>> placesGroupedByPlaceId = places.stream()
            .collect(Collectors.groupingBy(PlaceSummary::getPlaceId));

    Map<String, List<PlaceSummary>> placesGroupedByPlaceId =
          places.stream()
                .collect(Collectors.groupingBy(PlaceSummary::getPlaceId,
                                               LinkedHashMap::new,
                                               Collectors.toList()));

/**
 * I have written this code more generic, if you want then you can group based on any * 
 * instance variable , id, name etc via passing method reference.
class Student {
    private int id;
    private String name;
    public Student(int id, String name) {this.id = id;this.name = name;}
     * @return the id
    public int getId() {return id;}
     * @param id
     *            the id to set
    public void setId(int id) {this.id = id;}
     * @return the name
    public String getName() {return name;}
     * @param name
     *            the name to set
    public void setName(String name) {this.name = name;}
public class StudentMain {
    public static void main(String[] args) {
        List<Student> list = new ArrayList<>();
        list.add(new Student(1, "Amit"));
        list.add(new Student(2, "Sumit"));
        list.add(new Student(1, "Ram"));
        list.add(new Student(2, "Shyam"));
        list.add(new Student(3, "Amit"));
        list.add(new Student(4, "Pankaj"));
        Map<?, List<Student>> studentById = groupByStudentId(list,
                Student::getId);
        System.out.println(studentById);
       Map<?, List<Student>> studentByName = groupByStudentId(list,
                Student::getName);
        System.out.println(studentByName);
    private static <K, V> Map<?, List<V>> groupByStudentId(List<V> list,
            Function<V, K> keyFunction) {
        return list.stream().collect(
                Collectors.groupingBy(keyFunction, HashMap::new,

如何将List<P>中的元素分组到Map<K、List<V>>中，同时保持顺序？

3 个回答