Make me stop

Question

I can't make this thread stop. When the counter value is 20, I want to stop this thread. How do I do it?
public class StopThread extends Thread{
     public static void main(String []args){
               new StopThread().desing();
     public void desing() {
          Thread t=new Thread(new Threadclass("A"));
          t.start();
    class Threadclass extends Thread{
         String msg;
         public Threadclass(String msg){
              this.msg=msg;
         public void run() {
               try{
                    int counter=0;
                    while(true){
                         if(counter==30){
                              System.out.println("I'm sleeping");
                              Thread.sleep(10000);
                         if(counter==20){
                              System.out.println("making it stop----------");
                              Thread currentThread=Thread.currentThread();
                              Thread t=currentThread;
                              currentThread=null;
                              t.interrupt();
                         System.out.println("class counter "+ msg + "count value "+counter++);
               }catch(Exception e){
                    e.printStackTrace();
}

Answer 1

Thanks baftos idea really worked. I have also tried what jverd mentioned and was able to stop the thread. Please check whether this is a good way to stop it?
Another question: When/Why should we make a method static?
public class StopThread {
     public static void main(String []args){
          new StopThread().desing();
     public void desing() {
          Thread t=new Thread(new Threadclass("A"));
          t.start();
    class Threadclass implements Runnable{
         String msg;
         public Threadclass(String msg){
              this.msg=msg;
         public void run() {
       try{
          int counter=0;
          while(true){
               if(counter==30){
                    System.out.println("I'm sleeping");
                    Thread.sleep(10000);
               if(counter==20){
                    System.out.println("making it stop----------");
                     Thread currentThread=Thread.currentThread();
                       currentThread.interrupt();
                   if(currentThread.interrupted()){
                        return;
          System.out.println("class counter "+ msg + "count value "+counter++);
     }catch(Exception e){
          e.printStackTrace();
}