Change Object to Comparable in the following: ListReferenceBased, ListInterface, and Node.

class Node {

Object item; Node next; Node(Object newItem) { item = newItem; next = null; } // end constructor Node(Object newItem, Node nextNode) { item = newItem; next = nextNode; } // end constructor } // end class Node

public class ListReferencedBased implements ListInterface { // reference to linked list of items private Node head; private int numItems: // number of items in list // definitions of constructors and methods public ListReferencedBased() { numItems = 0; head = null; } // end default constructor public boolean isEmpty() { return numItems == 0; } public int size() { return numItems; } // end size }

private Node find(int index) { // // Locates a specified node in a linked list. // Precondition: index is the number of the desired // node. Assumes that 1 <= index <= numItems+1 // Postcondition: Returns a reference to the desired // node. // Node curr = head; for (int skip = 0; skip < index; skip++) { curr = curr.next; } // end for return curr; } // end find private Object get(int index) throws ListIndexOutOfBoundsException { if (index >= 0 && index < numItems) { // get reference to node, then data in node Node curr = find(index); Object dataItem = curr.item; return dataItem; } else { throw new ListIndexOutOfBoundsException(List index out of bounds on get); } // end if } // end get public void add(int index, Object item) throws ListIndexOutOfBoundsException { if (index >= 0 && index < numItems+1) { if (index == 0) { // insert the new node containing item at // beginning of list Node newNode = new Node(item, head); head = newNode; } else { Node prev = find(index-1); // insert the new node containing item after // the node that prev references Node newNode = new Node(item, prev.next); prev.next = newNode; } // end if numItems++; } else { throw new ListIndexOutOfBoundsException(List index out of bounds on add); } // end if } // end add public void remove(int index) throws ListIndexOutOfBoundsException { if (index >= 0 && index < numItems) { if (index == 0) { // delete the first node from the list head = head.next; } else { Node prev = find(index-1); // delete the node after the node that prev // references, save reference to node Node curr = prev.next; prev.next = curr.next; } // end if numItems; } else { throw new ListIndexOutOfBoundsException(List index out of bounds on remove); } // end if } // end remove public void removeAll() { // setting head to null causes list to be // unreachable and thus marked for garbage // collection head = null; numItems = 0; } // end removeAll } public interface ListInterface { // ******************************** // Inteface for the ADT List // ******************************** // list operations: public boolean isEmpty(); public int size(); public void add(int index, Object item) throws ListIndexOutOfBoundsException; public void remove(int index) throws ListIndexOutOfBoundsException; public Object get(int index) throws ListIndexOutOfBoundsException; public void removeAll(); } // end ListInteface