由于 HashMap.get(linkedList) 而引发 ClassCastException

Question

处理this 和类似条目，我创建了这段代码来从LinkedList 的HashMap 中删除一个条目。

    //Dbase in question
    HashMap<String, LinkedList<Item>> authorDbase = new HashMap<String, LinkedList<Item>>();
    TreeMap<String, Item> bookAisle = new TreeMap<String, Item>();
    ...
    Item book = bookAisle.get(title); // book == title's Item object reference
    authorDbase.get(author).remove(book); //Removes the item mapped to specified keyword

当在 main 中调用作为其一部分的函数时，我收到以下错误。

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to library.Item
        at library.Item.compareTo(Item.java:8)
        at java.util.TreeMap.getEntry(TreeMap.java:352)
        at java.util.TreeMap.remove(TreeMap.java:603)
        at library.Library.removeBook(Library.java:287)
        at Assignment3.removeBook(Assignment3.java:134)
        at Assignment3.main(Assignment3.java:118)

这是我如何实现Comparable<Item>

  8 public abstract class Item implements Comparable<Item>
  9 {
...
 26     @Override
 27     public int compareTo(Item i)
 28     {
 29 //      String title = this.title; DEBUG FLAG: delete maybe?
 30         return this.title.compareTo(i.title); //Returns a negative value if title < i.title, implements alphabetical order by title
 31     }

我的删除算法有什么问题？我知道它试图使用带有不正确参数的compareTo，这就是引发异常的原因，但我不应该能够从HashMap.get(listWanted) 的结果中调用LinkedList<Item> 的任何函数吗？为什么Comparable 是这里的一个因素？有人可以建议修复并纠正我对此的理解吗？

编辑

118行只是调用了删除算法所在的remove函数

118         removeBook(out, "The Curious Incident of the Dog in the Night-Time");

这里是removeBook

279     public boolean removeBook(String title)
280     {
281         Item book = bookAisle.get(title); // book == title's Item object reference
282         String author;
283         boolean successFlag = false;
284
285         if(book == null) //title doesn't exist in the bookAisle
286             successFlag = false;
287         else if (bookAisle.remove(book) != null) //It does exist and was removed
288             {
289                 //Deletes from author dbase
290                 author = book.getCreator(); //placed here to avoid potentially calling on a null object
291                 authorDbase.get(author).remove(book); //Removes the item mapped to specified keyword
292                 //Deletes from keyword dbase
293                 removeFromKDbase(book); //Removes object reference from KDbase
294                 successFlag = true;
295             }
296
297         return successFlag;
298     }

Answer 1

在第 287 行中，您正在调用 bookAisle.remove(book)，它有效地尝试从包含 String 作为键的 Treemap 中删除 Item。
(Treemap 的 Javadoc: "throws java.lang .ClassCastException 如果指定的键无法与地图中当前的键进行比较”。）

如果您将行更改为bookAisle.remove(title)，那么它应该可以工作。