Class 46: Implementing Lists with Nodes

Held: Thursday, 22 April 2004

Summary: Today we consider how to build lists from nodes, which are much like Scheme pairs. Such lists are typically called linked lists.

Related Pages:

• EBoard.
• ListNode.java.
• LinkedList.java.

Overview:

• Detour: Implementing isValid.
• Implementing Lists with Nodes.
• Basic Methods.
• Implementing Ordered Lists.
• Variants.

Detour: Invalidating Positions

• A number of methods officially invalidate all extant positions in the list.
• We assume that the client keeps track of which positions are valid and which are not.
• However, it is good programming practice to permit the client to verify preconditions when we can.
• How might we implement isValid?

Implementing Lists with Nodes

• The basic technique: Use something like a cons cell.

  /**
   * Nodes used to build linked lists.  In Saul's words, "These are
   * really structures."
   *
   * @author Samuel A. Rebelsky
   */
  class ListNode
    implements Position
  {
    /**
     * The contents of the node.  Should never be null.
     */
    Object contents;
  
    /**
     * The next node in the list.  Should be null for the last
     * element of the list.
     */
    ListNode next;
  
    /**
     * Build a new node with a subsequent list.
     */
    public ListNode(Object contents, ListNode next)
    {
      this.contents = contents;
      this.next = next;
    } // ListNode(Object, ListNode)
  
    /**
     * Build a new node with no successor.
     */
    public ListNode(Object contents)
    {
      this(contents, null);
    } // ListNode(Object)
  
  } // class ListNode
  

• What fields belong in our class?
• How do we implement the basic methods?
• add
• remove
• replace
• What about ordered lists?
• addToFront
• addToEnd
• addAfter

Variants

• Doubly-linked lists: Make removing before and moving backwards easier.
• Circularly-linked (doubly-linked) lists: Eliminate special cases.