问为什么这个同步方法不能像预期的那样工作？

EN

public class MyRunnable extends Main implements Runnable  {
  String name; // name of thread
  Thread t;

  MyRunnable (String threadname) {
      name = threadname;
      t = new Thread(this, name);
      t.start();
  }

  public void run() {
      try {
          for (int i=0;i<100000;i++) {
              extend(1);
          }
      } catch (InterruptedException e) {
          System.out.println("Thread interrupted.");
      }
      System.out.println("Thread " +  name + " exiting.");
  }

}

public class Main {
  private static List<Integer> numbers=new ArrayList<>();

  public synchronized void extend (int i) throws InterruptedException {
      numbers.add(i);
  }
  public synchronized static int getSize() {
      return numbers.size();
  }

  public static void main(String[] args) {
      MyRunnable t0=new MyRunnable("0");
      MyRunnable t1=new MyRunnable("1");
      MyRunnable t2=new MyRunnable("2");

      try {
          t0.t.join();
          t1.t.join();
          t2.t.join();
      } catch (InterruptedException e) {

      }
      System.out.println(getSize());
  }
}