cast error?

Hi:
I get the following runtime error when I try to run my Calendar program:

java.lang.ClassCastException
at Calendar.dupsTest(CalendarTest.java, Compiled Code)
at Calendar.print(CalendarTest.java, Compiled Code)
at CalendarTest.main(CalendarTest.java:20)
Exception in thread "main" Process Exit...

I know the problem occurs in the dupTest method but I am not sure how to correct it. Can someone help?

Thanks.

/**
Describes a calendar for a set of appointments.
@version 1.0
*/

import java.util.Vector;

public class CalendarTest
{  public static void main(String[] args)
   {
      Calendar markCalendar = new Calendar("Mark");
           
      markCalendar.addApp(new Appointment("June 1","3pm","4pm", "dentist"));
        markCalendar.addApp(new Appointment("June 2","3pm","4pm", "doctor"));
        markCalendar.print();
       
               
      Appointment toFind = new Appointment("June 2","3pm","4pm", "doctor");
      markCalendar.removeApp(toFind);    
     
      //markCalendar.addApp(new Appointment("June 2","3pm","4pm", "lawyer"));
      //markCalendar.print();
      markCalendar.print();
   }
}

/**
Describes a calendar for a set of appointments.
*/

class Calendar
{      
 
   /**
   Constructs a calendar for the person named.
   */  
   public Calendar(String aName)
   {  name = aName;
      appointments = new Vector();
   }
   
   
   /**  
   Adds an appointment to this Calendar.
   @param anApp The appointment to add.
   */
   public void addApp(Appointment anApp)
   {  
     
      appointments.add(anApp);
     
   }
   
   /**
   Removes an appointment from this Calendar.
   @param anApp The appointment to be removed.
   */
   
   public void removeApp(Appointment toFind)
   {      
      for ( int i = 0; i < appointments.size(); i++)
      {    
         if (((Appointment)appointments.get(i)).equals(toFind))
         {
         
             appointments.remove(i);
         }
      }
           
   }
   
   /**
   Tests for duplicate appointment dates.
   */
   public void dupsTest()
   {  
      for (int x = 0; x < appointments.size(); x++)
      {
         String cmp = (String)appointments.elementAt(x);
         for (int y = appointments.size()-1; y > x; y --)
         {
            if (cmp.equals((Appointment)appointments.elementAt(y)))
            System.out.println("duplicates");
         }
   
      }
   
   }  
   
   
   
   
 

   /**
   Prints the Calendar.
   */
   public void print()
   {  System.out.println(name + "               C A L E N D A R");
       
      System.out.println();
               
        System.out.println("Date   Starttime    EndTime   Appointment");
             
              
      for (int i = 0; i < appointments.size(); i++)
      {  Appointment nextApp =(Appointment) appointments.get(i);
         nextApp.print();
      }
     
      dupsTest();      
   }
   
   private Vector appointments;
   private String name;
   private Appointment theAppointment;
}



/**
Describes an appointment.
*/

class Appointment
{  
   public Appointment(String aDate,String aStarttime,String aEndtime, String aApp)
   {  date = aDate;
      starttime = aStarttime;
        endtime = aEndtime;  
      app = aApp;
   }
 
 /**
 Method to test whether on object equals another.
 @param otherObject  The other object.
 @return true if equal, false if not
*/
 
 public boolean equals(Object otherObject)
 {
      if (otherObject instanceof Appointment)
      {  Appointment other = (Appointment)otherObject;
                 
         return (date.equals(other.date) && starttime.equals(other.starttime)
                 && endtime.equals(other.endtime) && app.equals(other.app));
      }  
     
       else return false;
     
 }
 
   
     
   /**
   Prints the Date, Starttime, Endtime and a description of the
   appointment.
   */
   public void print()  
   {  System.out.println();
      System.out.println(date + "   " + starttime + "          " + endtime
           + "       " + app );
      System.out.println();
                 
   }
   
   private String date;
   private String starttime;
   private String endtime;
   private String app;

}