class LINKABLE [G]

feature

   item: G;              -- value held
   right: like Current;  -- Right neighbor

   putRight (other: like Current) is
         -- Put `other' to the right of current cell.
      do
         right := other
      ensure
         chained: right = other
      end;

end -- class LINKABLE


class BILINKABLE [G] inherit

        LINKABLE [G]
           redefine
              putRight
           end
           
feature -- Access

   left: like Current;   -- Left neighbor

   putRight (other: like Current) is
         -- Put `other' to the right of current cell.
      do
         right := other;
         if (other /= Void) then
            other.simplePutLeft (Current)
         end
       end;

    putLeft (other: like Current) is
           -- Put `other' to the left of current cell.
        do
           left := other;
           if (other /= Void) then
              other.simplePutRight (Current)
           end
        ensure
           chained: left = other
        end;

feature {BILINKABLE}

   simplePutRight (other: like Current) is 
         -- set `right' to `other'
      do
         right := other
      end;

   simplePutLeft (other: like Current) is
         -- set `left' to `other'
      do
         left := other
      end;

invariant

   rightSymmetry:
      (right /= Void) implies (right.left = Current);
   leftSymmetry:
      (left /= Void) implies (left.right = Current)
      
end -- class BILINKABLE

// Interface for node objects w/ values of type String

public interface NodeIfc<T>{

   // Returns following node.
   T getNext();

   // Hook up newNext to the right of this
   void setNext(T newNext);

   // Returns value stored in node
   String getValue();

   // Sets value stored in node to newVal
   void setValue(String newVal);
}
-----------------------------------------

// Interface for doubly-linked node objects w/ values of type String

public interface DbleNodeIfc<T> extends NodeIfc<T>{

   // Returns preceding node.
   T getPrevious();

   // Set node bfore this to be newPrevious
   void setPrevious(T newPrevious);
}

-----------------------------------------
// Interface for Ordered List of Strings, parameterized by the type of node.
// Note use of F-bounded polymorphism.
import java.util.Enumeration;

public interface OrdListIfc< N implements NodeIfc<N>>{

   // Return true iff val is in list
   boolean inList(String val);

   // Add newNode to list (in proper order).  Duplicates are OK.
   void add(N newNode);

   // Return an Enumeration which will go through all elements of list in order.
   Enumeration getEnumerator();

   // If val is in list, delete first copy and return true, else return false.
   boolean delete(String val);
}   

1. SNode for singly-linked nodes (that implements NodeIfc<SNode>).

2. DNode for doubly-linked nodes (that implements DbleNodeIfc<DNode>).
   Warning: The methods setNext and setPrevious in DNode must each set two links so the new elements are hooked up properly. You may find it easiest to add protected auxiliary methods to just set single links in order to get these to work properly.

3. OrdList<N implements NodeIfc<N>> for linked lists (that implements OrdListIfc<N>).

   The class OrdList should result in a singly linked list if it is instantiated with SNode, and a doubly-linked list if instantiated with DNode. (Note: SNode and DNode cannot be subclasses.)

4. Depending on the instantiation, OrdList can represent either singly or doubly-linked lists, but doubly-linked lists usually provide more specialized implementations and methods. Write class DOrdList<N implements DbleNodeIfc<N>> which extends OrdList and adds a tail instance variable and method

      // Return Enumeration which goes through elements of list in reverse order.
      public Enumeration getRevEnumerator();
   

   You will also have to override methods add and delete to ensure that the tail instance variable gets updated properly. See if you can write them so that you call the inherited method from the superclass and then simply execute a few extra instructions to make sure that tail has the proper value.