Generic Java - Linked List

dear all,

I done the following LinkedList and Node classes. I have problem coding the sorting portion (the bold line).
I keep getting

cannot find symbol
method compareTo(E)
class: java.lang.object

what should I do to make my coding works??

Thanks in advance

import java.lang.Comparable;
abstract class LinkedList<E extends Comparable<E>>
// Representation of a linked list of objects of type E.
{
    protected int numItems;       // number of elements in list
    protected LLNode<E> head;     // pointer to head of list
 
    public LinkedList()
    // Default constructor - create new empty linked list.
    {
        numItems = 0;
        head = null;
    } 
 
    // Accessor methods.
    public LLNode<E> getHead() { return head;           }
    public int size()          { return numItems;       }
    public boolean isEmpty()   { return (head == null); }
 
    public void setHead (LLNode<E> newHead) 
    {
        head = newHead;
    } 
 
    public void setNumItems (int newNumItems)
    {
        numItems = newNumItems;
    } 
    
    public void insert(E var)
    {
        LLNode<E> newNode = new LLNode<E> (var); 
        setNumItems(size()+1);
        if(isEmpty())
            setHead(newNode);
        else
        {   LLNode<E> prev=null;
            for(LLNode<E> curr=getHead(); curr != null; prev = curr, curr=curr.getNext()) 
            {
                if(curr.getItem().compareTo(var) > 0) 
                {
                    // insert before curr node
                    newNode.setNext (curr);
                    if (prev != null)
                        prev.setNext (newNode);
                    else
                        setHead (newNode);
                    
                    return;
                } 
            } 
            // insert at end of list
            prev.setNext (newNode);
        }
    }
    
 
 
 
 
 
import java.lang.Comparable;
class LLNode<E>
// Representation of a node containing an object of type E in a linked list.
{
    private E item;
    private LLNode<E> next;
 
    public LLNode (E newItem)
    // Constructor - create new node containing newItem.
    {
        item = newItem;
        next = null;
    } // end constructor
 
    public LLNode (E newItem, LLNode<E> newNext)
    // Constructor - create new node containing newItem and pointing to
    // newNext.
    {
        item = newItem;
        next = newNext;
    } // end constructor
 
    // Accessor methods.
    public E getItem()         { return item; }
    public LLNode<E> getNext() { return next; } 
 
    public void setItem (E newItem)  { item = newItem; }
    public void setNext (LLNode<E> newNext)
    {
        next = newNext;
    }
    
    
} // end LLNode class

[ September 22, 2005: Message edited by: Timothy Leong ]

by the way,

is Parametric Polymorphism under Java Collections Framework?? Thanks

The getItem() method returns an E object, not a LLNode<E> object. The compareTo() method is not available in the E object.

In any case, I wouldn't know how to fix it, as I don't know how the compareTo() method is supposed to work. It looks like all the LLNode<E> class' compareTo() method does is return a -2.

Henry

hi guys,

I have edited my codings

public class BankDatabase extends LinkedList<Customer>
{
.....
}

however I keep getting "Type parameter Customer is not within its bound"

what's the problem here?? thanks
[ September 22, 2005: Message edited by: Timothy Leong ]

up...

any1 know wat's wrong with the codes?? Thank you

You are not implementing comparable<T> on the node. I think you should reduce the clutter and get a tiny example working first.
[ September 23, 2005: Message edited by: Rick O'Shay ]

i tried casting it using Comparator but still receive the same error.
Do you mean i should compare with nodes instead of the object?

I am trying to compare the E objects together and let them call their compareTo methods which I have already specified in their own classes

[ September 24, 2005: Message edited by: Timothy Leong ]
[ September 24, 2005: Message edited by: Timothy Leong ]

Originally posted by Timothy Leong:
hi guys,

I have edited my codings

public class BankDatabase extends LinkedList<Customer>
{
.....
}

however I keep getting "Type parameter Customer is not within its bound"

what's the problem here?? thanks

[ September 22, 2005: Message edited by: Timothy Leong ]

This one is easy
it should be
public class BankDatabase<Customer> extends LinkedList<Customer>

it says type parameter out of bounds because the Customer type is not avaiable to the BankDatabase. What you've done is just made a "raw" type extend a "generic" type while what you intended was "generic" type extending "generic" type. Hope that helps. (I have not looked into your first problem yet)

Originally posted by Timothy Leong:
public void insert(E item) { ... { Comparable<E> tmpItem = (Comparable<E> ) curr.getItem(); if(tmpItem.compareTo(newNode.getItem())> 0) { ... } } // insert at end of list prev.setNext (newNode); } }

[ September 24, 2005: Message edited by: Timothy Leong ]

[ September 24, 2005: Message edited by: Timothy Leong ]

Try this
public <E extends Comparable<E>> void insert(E o)
and get rid of that cast, I think that should do the job
if it doesn;t work, try the case E temp= curr.getItem(); //this should not have any significant effect
and in the worst,
Comparable<?> temp = (Comparable<?>

curr.getItem() //though the cast is unecessary.