ClassCastException in list implementation?

Question

I am trying to implement a list ADT in Java.I followed a book on Data Structures by Goodrich.

I created 2 interfaces called Position:

public interface Position {
    Object element()throws InvalidPositionException;

}

and List which has method declaration of lists like first(), last(), etc.

Now, i have a class called DNode.java which implements Position and has methods to assign the previous and next elements for a particular node.

And finally, I have NodeList.java which implements the List interface.

In this, there is a insertBefore() method that takes Position and Object as input and makes changes to the list to insert the node before a particular position.

Now, this method is called from the ListMain class which is the main class.

    Object p = (Object) br.readLine();
    nl.insertAfter((Position)p, element);
    nl.display();

Basically, I have taken an input from the user in the form of a string, converted it to type Object and then converted it to type Position. When I do so, I get a class cast exception that says

java.lang.String cannot be cast to ListPackage.Position

Can someone please tell me how to do the appropriate type casting. Or is my approach wrong and should I be doing something else?

I am new to data structures. So, any help will be appreciated. Thank you very much in advance.

What is br? If that's a BufferedReader, then obviously you read String, and it can't be casted to a Position. — Andrey Nudko
– Andrey Nudko, Commented May 9, 2012 at 19:57

Madbreaks · Accepted Answer · 2012-05-09 19:56:14Z

3

You can't arbitrarily cast objects to other types in all cases. Consider writing a constructor for Position that takes the string value(s) the user provides as parameters.

answered May 9, 2012 at 19:56

Madbreaks

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Attila · Accepted Answer · 2012-05-09 19:58:25Z

2

As the error says you are trying to say that a String is a Position, which it is not. Instead you need to create a Position from the String:

String s = br.readLine();
Position p = new DNode(s);

And you will have to implement the logic of how to convert a String into a DNode object in the DNode class's constructor

answered May 9, 2012 at 19:58

Attila

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2 Comments

Lavanya Mohan Over a year ago

I created a constructor in DNode that took string as parameter. But I am not able to set the prev and next fields to anything other than null (which I do not want). What should I do?

Attila Over a year ago

You need to add the prev/next links externally by calling say a setNext()/setPrev() method. Who is doing the calling is another question: you could do it directly from main(), but since you have a NodeList class already, you could call its insertNode() method. If you do the latter, you can specify the position via an index or an Iterator (if your List supports that) -- it's up to you, based on what makes sense (see if there is anything similar in the List interface already; if yes, implement/use that method)

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