Problem with synchronisation

Question

i have created an applet with two buttons and have brought in concept of threads
and when i click one button a thread starts and it starts printing
the numbers and if i press another button another
thread starts and it also start printing
but my requirement is when i press the button
until the printing by one thread is completed even
when other button is pressed it should not start printing.
i tried doing it by synchronsing the particular method which does
the printing but still pressing the other button also bought the numbers
in the picture
can anybody tell me why is this that even after synchronizing is done for that method the pressing of other button starts printing...
The code is given below
public class Thread_ex extends Applet implements ActionListener{
     public Canvas display;
     boolean kd =true;
     public Thread_ex() throws HeadlessException {
          super();
     Counter k;
     Counter k1;
     int i=0;
     * @param args
     public void init()
          /* Make on/off buttons for threads */
               Button a = new Button("ON/OFF 1");
               add(a);
               Button b = new Button("ON/OFF 1");
               add(b);
               a.addActionListener(new Thread_ex() );
               b.addActionListener(new Thread_ex() );
          public void actionPerformed(ActionEvent arg0) {
               // TODO Auto-generated method stub
               k = new Counter();
               k.start();
public class Counter extends Thread {
//private volatile int x =0;
     public Counter() {
          super();
          // TODO Auto-generated constructor stub
     public void run()
          set();
     public synchronized void set(){
          int i=0 ;
          while(i<50)
          long currentTimeMillis = System.currentTimeMillis();
          long changeTime = 0;
          while((System.currentTimeMillis() - currentTimeMillis) <= 1000){
          //changeTime = ;
          //     System.out.println(System.currentTimeMillis());
          System.out.println(i);
          i++;
}

Answer 1

You are creating new instances of Counter and they are all syncrhonizing on themselves.
Either synchronize the set() method internally on Counter.class, or change Counter to a Runnable of which there is only one instance, and start a new Thread with that as the Runnable every button press, and leave the set() method alone.