Java - GUI mit zwei Buttons die parallel, in separaten Threads laufen

package multithread;
 
import java.awt.event.*;
import java.io.*;
import java.util.*;
import javax.swing.*;
import javax.swing.border.EmptyBorder;
 
public class RandomGetDataFromFileIO extends JFrame {
 
      private static final long serialVersionUID = 1L;
      private Thread th;
      private JButton btnStart;
      private JButton btnStop;
      private JTextField tfn;
      private JTextArea tfa;
 
      public void initGUI() {
            JPanel p = new JPanel();
            p.setBorder(new EmptyBorder(5, 5, 5, 5));
            setContentPane(p);
            p.setLayout(null);
            btnStart = new JButton("Start");  
            btnStart.setBounds(50, 30, 80, 30);
            p.add(btnStart);
            btnStart.addActionListener(new ActionListener() {
                  public void actionPerformed(ActionEvent e) {
                        getDataFromFile();
                        btnStart.setEnabled(false);
                        btnStop.setEnabled(true);
                  }
            });
            btnStop = new JButton("Stop");  
            btnStop.setBounds(150, 30, 80, 30);
            p.add(btnStop);
            btnStop.addActionListener(new ActionListener() {
                  public void actionPerformed(ActionEvent e) {
                        tfa.append(tfn.getText() + "\n");  
                  }
            });
            btnStop.setEnabled(false);
            tfn = new JTextField();
            tfn.setBounds(50, 80, 200, 20);
            p.add(tfn);
            setSize(300, 300);
            setVisible(true);
            tfa = new JTextArea();
            JScrollPane s = new JScrollPane();
            s.setBounds(50, 120, 200, 100);
            s.setViewportView(tfa);
            p.add(s);
      }
 
      public RandomGetDataFromFileIO() {
            initGUI();
      }
 
      public void getDataFromFile() {
            th = new Thread(new Runnable() {
                  public void run() {
                        BufferedReader in;
                        String data = "\n", l;  
                        try {
                             in = new BufferedReader(new FileReader("test1.txt"));  
                             while (true) {
                                   while ((l = in.readLine()) != null) {
                                         data += l;
                                   }
                                   StringTokenizer t = new StringTokenizer(data, "&");  
                                   if (t.hasMoreTokens() == false) {
                                         in = new BufferedReader(new FileReader("test.txt"));  
                                   }
                                   while (t.hasMoreTokens()) {
                                         tfn.setText(t.nextToken());
                                         Thread.sleep(70);
                                   }
                             }
                        } catch (Exception e) {
                             e.printStackTrace();
                        }
                  }
            });
            th.start();
      }
 
      public static void main(String args) {
            new RandomGetDataFromFileIO();
      }
}

Hallo&dies&ist&ein&Beispiel&Skript&das&lediglich&der&Demonstration&dient