Question

我试图在HashMap中存储和访问2种不同类型的List，并在调用返回HashMap的方法时对其进行访问，但是我收到此错误： (new int[] {1,2, 3}).Contains(1)
该方法如下所示：

The constructor ArrayList<JournalArticle>(Object) is undefined

当我调用该方法并尝试将它们各自的值存储到新列表中时，我得到了之前注释过的错误：

public static HashMap<String, Object> getJournalArticles(long groupId) throws NumberFormatException, SystemException{
    List<JournalArticle> journalArticles = JournalArticleLocalServiceUtil.getStructureArticles(groupId);
    List<String> allJournalArticleIds = new ArrayList<String>();
    for (JournalArticle journalArticle : journalArticles) {
        allJournalArticleIds.add(journalArticle.getArticleId());
    }

    HashMap<String, Object> mapArticles = new HashMap<String,Object>();
    mapArticles.put("journalArticles", journalArticles);
    mapArticles.put("allJournalArticleIds", allJournalArticleIds);

    return mapArticles;
}

怎么了？怎么解决？

Answer 1

我将使用编写的类来保存此信息（使用Pair<L, R>之类的书可能会更快）：

class ArticleHolder {
    private List<JournalArticle> journalArticles;
    private List<String> allJournalArticleIds;

    public ArticleHolder(List<JournalArticle> journalArticles,
        List<String> allJournalArticleIds) {
        this.journalArticles = journalArticles;
        this.allJournalArticleIds = allJournalArticleIds;
    }

    //getters + setters
}

并更改您的方法：

public static ArticleHolder getJournalArticles(long groupId) 
    throws NumberFormatException, SystemException {

    List<JournalArticle> journalArticles = 
                JournalArticleLocalServiceUtil.getStructureArticles(groupId);
    List<String> allJournalArticleIds = new ArrayList<String>();

    for (JournalArticle journalArticle : journalArticles) {
        allJournalArticleIds.add(journalArticle.getArticleId());
    }

    return new ArticleHolder(journalArticles, allJournalArticleIds);
}

此外：您对new ArrayList<JournalArticle>(...)的调用表明您打算创建新数组列表实例（假设代码可以编译）。即使您将地图值键入为Collection个对象，也不必这样做。

Answer 2

恕我直言，快速解决方案是将mapArticles的类型更改为此HashMap<String, List<?>>，然后：

List<JournalArticle> allArticles = new ArrayList<JournalArticle>((Collection<JournalArticle>)mapArticles.get("journalArticles"));
List<String> allJournalArticleIds = new ArrayList<String>((Collection<String>)mapArticles.get("allJournalArticleIds"));

因为ArrayList构造函数仅支持以下选项：

 new ArrayList<T>();
 new ArrayList<T>(int capacity);
 new ArrayList<T>(Collection<? extends T> collection);

Object在编译时不是集合。

如何在HashMap中存储和访问2种不同类型的列表

2 个答案: