If this is possible, how can I change what a method does after I might have created an instance of that class and wish to keep the reference to that object but override a public method in it's class' definition?

Here's my code:

    package time_applet;

    public class TimerGroup implements Runnable{

        private Timer hour, min, sec;
        private Thread hourThread, minThread, secThread;

        public TimerGroup(){
            hour = new HourTimer();
            min = new MinuteTimer();
            sec = new SecondTimer();
        }


        public void run(){
            hourThread.start();
            minThread.start();
            secThread.start();
        }

/*Please pay close attention to this method*/
        private Timer activateHourTimer(int start_time){
            hour = new HourTimer(start_time){

                public void run(){
                    while (true){

                        if(min.changed)//min.getTime() == 0)
                            changeTime();

                    }

                }
            };
            hourThread = new Thread(hour);
            return hour;
        }

        private Timer activateMinuteTimer(int start_time){

            min = new MinuteTimer(start_time){
                public void run(){
                    while (true){

                        if(sec.changed)//sec.getTime() == 0)
                            changeTime();

                    }

                }
            };
            minThread = new Thread(min);    
            return min;
        }

        private Timer activateSecondTimer(int start_time){
            sec = new SecondTimer(start_time);
            secThread = new Thread(sec);
            return sec;
        }

        public Timer addTimer(Timer timer){
            if (timer instanceof HourTimer){
                hour = timer;
                return activateHourTimer(timer.getTime());
            }
            else if (timer instanceof MinuteTimer){
                min = timer;
                return activateMinuteTimer(timer.getTime());
            }
            else{
                sec = timer;
                return activateSecondTimer(timer.getTime());        
            }
        }
    }

So for example in the method activateHourTimer(), I would like to override the run() method of the hour object without having to create a new object. How do I go about that?