CODEWITHSUNDEEP
javalinked-listhashmapcomparable
Legion Daeth
Legion Daeth

Reputation: 329

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

Working off this and similar entries I created this piece of code to remove an entry from a HashMap of LinkedList.

    //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

When the function this is a part of is called in main I get the following error.

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)

Here is how I implemented 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     }

What's wrong with my deleting algorithm? I understand that it's trying to use compareTo with an incorrect argument and that's what is throwing the exception but shouldn't I be able to call any function of LinkedList<Item> from the result of a HashMap.get(listWanted)? Why is Comparable a factor here? Could someone please suggest a fix and correct my understanding of this?

EDIT

Line 118 just calls the remove function that the deleting algorithm is in

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

Here is 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     }

Upvotes: 0

Views: 597

Answers (1)

Nevay
Nevay

Reputation: 794

In line 287 you are calling bookAisle.remove(book), which effectively tries to remove an Item from a Treemap that does contain String as key.
(Javadoc of Treemap: "throws java.lang.ClassCastException if the specified key cannot be compared with the keys currently in the map".)

If you change the line to bookAisle.remove(title), then it should work.

Upvotes: 2

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