ClassCastException problem...

Question

Ok, here's my problem. The method in question, processFindBookTransaction, is supposed to search go into a binary search tree and find a "book" with a certain key. Here's the code for that method:
  private static Book processFindBookTransaction(Transaction aTransaction)
    Book aBook = (Book) library.findByKey(aTransaction.getISBN());
    if (aBook != null)
      System.out.println(aTransaction.getISBN() +
                         " found: " + aBook );
    else
      System.out.println(aTransaction.getISBN() +
                         " cannot be found");
    return aBook;
  }The problem is in the line reading:
Book aBook = (Book) library.findByKey(aTransaction.getISBN());Here is the code for the method that is accessed in the binary search tree (library being the binary search tree object):
  public Object findByKey(Comparable aKey)
    Node currentNode = rootNode;
    Node parentNode = null;
    boolean found = false;
    while(!found && currentNode != null)
      if(aKey.compareTo(currentNode.getNodeKey()) < 0)
        parentNode = currentNode;
        currentNode = (Node)currentNode.getLeftChild();
      else
        if(aKey.compareTo(currentNode.getNodeKey()) > 0)
          parentNode = currentNode;
          currentNode = (Node) currentNode.getRightChild();
        else
          found = true;
          return currentNode;
    }It throws a ClassCastException, and I can't figure out why. For some reason, it's having a problem explicitly casting an object of type Object to one of type Book. This problem is beyond my comprehension, so I hope someone can help. I appreciate any input. Thank you.

Answer 1

It's happening at runtime, unfortunately. From what I can follow, it start by taking a Transaction object, and just pulling a String from the attributes of it. Then, it takes the String and gives it to the findByKey method, which takes it in as a Comparable. The Comparable is then used in a series of comparisons, narrowing down the potential Nodes for a match. Then, at the end of the find method, it returns a Node, which is then attempted to be cast as a Book. Now, I don't know if it casts it as an Object before returning it, or if it's just giving back the Node to be cast as the Book. (I'm not that fluent in Java to understand the specifics)
By the way, I accidentally cut off the last 2 lines from the find method. The first one read return parentNode; and the second line was just the closing bracket for the whole method.