我有一个代码，想知道里面th和xx的变化规律

Question

import javax.swing.*;
import java.awt.event.*;//事件处理类
//符合标准,实现了接口ActionListener
//包含有方法actionPerformed()
public class DMLst implements ActionListener,Runnable
{
    JFrame frm;
    JPanel p;    JButton btn;    public void run()
    {
       int k=1;
       while(k<=20)
       {
          System.out.println(k);
          k++;
       }
    }    public DMLst()
    {
       frm=new JFrame();
       p=new JPanel();       btn=new JButton("OK");       //btn.addActionListener(new DMLst());
       btn.addActionListener(this);
       //this相当于是new DMLst(),指当前类的对象
       p.add(btn);       frm.getContentPane().add(p);
    frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frm.setSize(300,300);
    frm.setVisible(true);
    }    public static void main(String args[])
    {
       new DMLst();
    }
    public void actionPerformed(ActionEvent evt)
    {
         System.out.println("ABCDEFG");         //Thread th=new Thread(new DMLst());
         Thread th=new Thread(this);
         th.start();         System.out.println("123456");         Thread xx=new Thread(this);
         xx.start();         System.out.println("The END.");
    }
}