Demonstration of Linked List

//****************************
//LinkedList.java
//****************************
// Modified to support backwards traversal of the list.
// Additions and modifications are marked by ***.

public class LinkedList
{
  private ListItem start;              // First ListIem in the list.
  private ListItem end;                // Last ListIem in the list.
  private ListItem current;            // The current item for iterating.

  // Constructor to create a list containing one object:
  public LinkedList(Object item)
  {
    start = new ListItem(item);           // item is the start
    current = end = start;                // as well as the end and current.
  }

  // Construct a linked list from an array of objects:
  public LinkedList(Object[] items)
  {
    // Create a one item list:
    start = new ListItem(items[0]);       // First item is the start
    end = start;                          // as well as the end.

    // Now add the other items:
    for(int i = 1; i < items.length; i++)
      addItem(items[i]);
  }

  // Add an item object to the list:
  public void addItem(Object item)
  {
    ListItem newEnd = new ListItem(item); // Create a new ListItem.
    end.setNext(newEnd);                  // Set next variable for old end as new end.
    newEnd.setPrevious(end);              // So previous for new item.          ***
    current = end = newEnd;               // Store new item as end and current. ***
  }

  // Get the first object in the list:
  public Object getFirst()
  {
    current = start;
    return start.getObject();
  }

  // Additional method to get the last object in the list:     ***
  public Object getLast()
  {
    current = end;
    return end.getObject();
  }

  // Get the next object in the list:
  public Object getNext()
  {
    current = current.getNext();    	  // Get the reference to the next item.
    return current == null ? null : current.getObject();
  }

  // Additional method to get the previous object in the list:   ***
  public Object getPrevious()
  {
    current = current.getPrevious();   // Get the reference to the previous item.
    return current == null ? null : current.getObject();
  }
}


//****************************
//ListItem.java
//****************************
// Modified to support backwards traversal of the list.
// Additions and modifications are marked by ***.

public class ListItem
{
  ListItem next;             // Refers to next item in the list.
  ListItem previous;         // Refers to the previous item.       ***
  Object item;               // The item for this ListItem.

  // Constructor: 
  public ListItem(Object item)
  {
    this.item = item;        // Store the item.
    next = previous = null;  // Set next and previous to null.      ***
  }

  // Set the pointer to the next ListItem:
  public void setNext(ListItem next)
  {
    this.next = next;        // Store reference to the next item.
  }
  
  // Additional method to set the pointer to the previous ListItem:  ***
  public void setPrevious(ListItem previous)
  {                                                               
    this.previous = previous; // Store reference to the previous item. 
  }
 
  // Get the next item in the list:
  public ListItem getNext()
  {
    return next;
  }

  // Additional method to get the previous item in the list:         ***
  public ListItem getPrevious()
  {
    return previous;
  }

  // Get the object for this item:
  public Object getObject()
  {
    return item;
  }

  // Return class name & object:
  public String toString()
  {
    return "ListItem " + item;
  }
}


//****************************
//Point.java
//****************************

public class Point
{
  double x;
  double y;

// Constructors:

  public Point()
  {
    x = 0.0;
    y = 0.0;
  }

  // Construct a Point from its coordinates:
  public Point(double x, double y)
  {
    this.x = x;
    this.y = y;
  }

  // Construct a Point from another Point:
  public Point(Point point)
  {
    x = point.x;
    y = point.y;
  }

  // Method to return a point defined relative to this
  // point in coordinates:
  public Point add(Point z)
  {
    return new Point(x+z.x,y+z.y);
  }

  // Overrides the method inherited from Object:
  public String toString()
  {
    return "Point: " + x + "," + y;
  }
}

//****************************
//PolyLine.java
//****************************

// MODIFIED TO OUTPUT THE POLYLINE POINTS IN REVERSE ORDER.
public class PolyLine
{
  LinkedList polyline;      			// The linked list of points.

  // Construct a polyline from an array of coordinate pairs:
  public PolyLine(double[][] coords)
  {
    Point[] points = new Point[coords.length];  // Array to hold points.

    // Create points from the coordinates:
    for(int i = 0; i < coords.length ; i++)
      points[i] = new Point(coords[i][0], coords[i][1]);

    // Create the polyline from the array of points:
    polyline = new LinkedList(points); 
  }

  // Construct a polyline from an array of points:
  public PolyLine(Point[] points)
  {
    polyline = new LinkedList(points);      	// Create the polyline.
  }
  
  // Add a Point object to the list:
   public void addPoint(Point point)
   {
    polyline.addItem(point);                 	// Add the point to the list.
   }

  // Add a point from a coordinate pair to the list:
   public void addPoint(double x, double y)
   {
     polyline.addItem(new Point(x, y));    	 // Add the point to the list.
  }

  // Output the polyline in reverse order:
  public void show()
  {
    System.out.println("Polyline points are:");

    // Set the 1st point as start:
    Point nextPoint = (Point)polyline.getLast();                 //  ***   

    // Output the points:
    while(nextPoint != null)
    {
      System.out.println(nextPoint);             // Output the current point.
      nextPoint = (Point)polyline.getPrevious(); // Get the next point.
    }
  }
}

//****************************
//TryPolyLine.java
//****************************

public class TryPolyLine
{
   public static void main(String[] args)
   {
      // Create an array of coordinate pairs:
      double[][] coords = { {1., 1.}, {1., 2.}, { 2., 3.},
                            {-3., 5.}, {-5., 1.}, {0., 0.} };

      // Create a polyline from the coordinates and display it:
      PolyLine polygon = new PolyLine(coords);
      polygon.show();

      // Add a point and display the polyline again:
      polygon.addPoint(10., 10.);
      polygon.show();

      // Create Point objects from the coordinate array:
      Point[] points = new Point[coords.length];
      for(int i = 0; i < points.length; i++)
         points[i] = new Point(coords[i][0],coords[i][1]);

      // Use the points to create a new polyline and display it:
      PolyLine newPoly = new PolyLine(points);
      newPoly.show();
   }
}