Java 8 流维护特定的映射键顺序从分组操作返回

Question

我有一个包含 TitleIsbnBean 对象集合的列表。我编写了以下代码片段，按以下学习区域类型按此集合分组。

titleListByLearningArea = nonPackageIsbnList.stream()
      .collect(groupingBy(TitleIsbnBean::getKeylearningarea,
                          LinkedHashMap::new,Collectors.toList())

              );

但我想在上述流的 map 返回中保留以下特定顺序。

titleListByLearningArea.put("Commerce", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("English", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Health & PE", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Humanities", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Mathematics", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Music & the Arts", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Science", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Others", new ArrayList<TitleIsbnBean>() {});

但是一旦我使用流按集合分组，我就会得到不同的顺序。使用流式分组时如何维护特定顺序。

class TitleIsbnBean {

  private String titleName;
  private String isbn;
  private int status;
  private String keylearningarea;

  public TitleIsbnBean(String titleName, String isbn, int status, String keylearningarea){
    super();
    this.titleName = titleName;
    this.isbn = isbn;
    this.status = status;
    this.setKeylearningarea(keylearningarea);
  }

}

ArrayList<TitleIsbnBean> nonPackageIsbnList = new ArrayList<>();
Map<String,List<TitleIsbnBean>> titleListByLearningArea = new LinkedHashMap<>();

titleListByLearningArea.put("Commerce", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("English", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Health & PE", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Humanities", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Mathematics", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Music & the Arts", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Science", new ArrayList<TitleIsbnBean>() {});
titleListByLearningArea.put("Others", new ArrayList<TitleIsbnBean>() {});

titleListByLearningArea = nonPackageIsbnList.stream()
.collect(Collectors.groupingBy(TitleIsbnBean::getKeylearningarea,
                                LinkedHashMap::new,Collectors.toList()));

Answer 1

考虑到您需要 Map 的所需键顺序您正在收集，您可以收集到TreeMap与 Comparator基于 index指定如:

Collection<TitleIsbnBean> nonPackageIsbnList = .... //initialisation
List<String> orderedKeys = List.of("Commerce", "English", "Health & PE", "Humanities",
        "Mathematics", "Music & the Arts", "Science", "Others");

Map<String, List<TitleIsbnBean>> titleListByLearningArea = nonPackageIsbnList.stream()
        .collect(Collectors.groupingBy(TitleIsbnBean::getKeylearningarea,
                () -> new TreeMap<>(Comparator.comparingInt(orderedKeys::indexOf)),
                Collectors.toList()));