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.

Answer 1

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