Listing 4. Model

import java.util.*;
/**
 * EXTRACTS FROM JPModel, the data model for JabberPoint.
 * It keeps track of all the slides in the presentation.
 */
public class JPModel extends Observable {
   /** the Vector of Slides */
   Vector show = new Vector();
   /** The currently-displayed page */
   int pageNumber = 0;
   /** Append a slide to the presentation */
   public void append(Slide s) {
      show.addElement(s);
   }
   public Slide getPage() {
      return (Slide)show.elementAt(pageNumber);
   }
   /* Here are some methods used 
    * in the Controller(s) to control
    * what part of the data the view displays:
    */
   /** setPage(int pnum) sets the current 
    * page(slide) and notifies all the view(s)
    * of the current page
    */
   public void setPage(int i) {
      if (i<0 || i>=show.size())
         throw new IllegalArgumentException();
      pageNumber = i;
      // for the Observers (required!)
      setChanged();
      // tell the observers the current page
      notifyObservers(getPage());
   }
   /** Move to the previous page, unless 
    *  at beginning */
   public void prevPage() {
      if (pageNumber > 0)
         setPage(pageNumber-1);
   }
   // nextPage() is the same but in reverse
}