HashMap.get(linkedList) 引发的 ClassCastException

Question

处理 this 和类似的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 中调用它所属的函数时，出现以下错误。

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)

我是这样实现的 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？我知道它试图将 compareTo 与不正确的参数一起使用，这就是抛出异常的原因，但我不应该能够从 HashMap.get(listWanted) 的结果中调用 LinkedList<Item> 的任何函数？为什么 Comparable 在这里是一个因素？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)，它有效地尝试从确实包含 String 作为键的 Treemap 中删除 Item。
（树形图的 Javadoc："throws java.lang.ClassCastException if the specified key cannot be compared with the keys currently in the map"。）

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

HashMap.get(linkedList) 引发的 ClassCastException

ClassCastException being thrown as a result of HashMap.get(linkedList)

java

linked-list

hashmap

comparable