ช่วยหน่อยคะ รับค่าจาก Java GUI แล้ว save ข้อมูลลง .txt แบบไม่ลบของเก่าทำไงอะคะ

import java.awt.*; 						
import javax.swing.*;
import java.io.*;
import java.awt.event.*; 
class sum1{
  String id;
  double pb,pf,ps;
}
public class MainWindow extends JFrame {
  float pt,pb,pf,ps;
  JPanel panel;
  JLabel l1;
  JButton addbtn, calbtn, showbtn,closebtn;
  Font fn = new Font("Angsana New", Font.BOLD, 20);
  Font fn1 = new Font("Angsana New", Font.BOLD, 30);
  AddPanel x;
  public  MainWindow() {
    setTitle("หน้าหลัก");
    setSize(320, 250);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    
    buildPanel();
    add(panel);
    setVisible(true);
  }
  private void buildPanel() {
    l1 = new JLabel("โปรแกรมจัดการข้อมูลค่าใช้จ่าย",SwingConstants.CENTER);
    Icon ani4 = new ImageIcon("i4.jpg");
    Icon ani2 = new ImageIcon("i2.jpg");
    Icon ani3 = new ImageIcon("i3.jpg");
    Icon ani5 = new ImageIcon("i5.jpg");
    addbtn =   new JButton(" รับข้อมูลค่าใช้จ่าย ",ani2);
    calbtn =  new JButton("   คำนวณค่าใช้จ่าย ",ani4);
    showbtn =  new JButton("   แสดงค่าใช้จ่าย ",ani5);
    closebtn = new JButton(" ปิดโปรแกรม ",ani3);
    l1.setFont(fn1);
    l1.setForeground(Color.BLUE);
    addbtn.setFont(fn);
    calbtn.setFont(fn);
    showbtn.setFont(fn);
    closebtn.setFont(fn);
    addbtn.addActionListener(new ButtonListener());
    calbtn.addActionListener(new ButtonListener());
    showbtn.addActionListener(new ButtonListener());
    closebtn.addActionListener(new ButtonListener());	
    panel = new JPanel();
    panel.setLayout(new GridBagLayout());
    panel.setBackground(Color.WHITE);
    x = new AddPanel();
    
    x.addItem(panel,l1,1,0,1,1,GridBagConstraints.CENTER);
    x.addItem(panel,addbtn,1,2,1,1,GridBagConstraints.CENTER);
    x.addItem(panel,calbtn,1,3,1,1,GridBagConstraints.CENTER);
    x.addItem(panel,showbtn,1,4,1,1,GridBagConstraints.CENTER);
    x.addItem(panel,closebtn,1,5,1,1,GridBagConstraints.CENTER);
  }
  private class ButtonListener implements ActionListener {
    @Override
    public void actionPerformed(ActionEvent e) {
      if (e.getSource()==addbtn) { //เพิ่มสินค้า
        Adddata a = new Adddata();
        a.setSize(400, 350);
        a.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE); //DISPOSE ให้ปิดหน้าจอปัจจุบันแล้วกลับมาหน้าจอปัจจุบัน  
             
      }
      if (e.getSource()==calbtn) {//คำนวณ
   
  }
      if (e.getSource()==showbtn) {//แสดง
        
      }
      if(e.getSource()==closebtn)  { //ปิดโปรแกรม
          System.exit(0);
      }
    }
  }
     public static void main(String[] args)  {
     MainWindow m = new MainWindow();
     m.setSize(500, 350);
     m.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //EXIT ออกจากโปรแกรม
     m.setVisible(true);
   }
} 

import java.awt.*; 		
import javax.swing.*;	
import java.awt.event.*; 
import java.io.*;
import java.text.DecimalFormat;

public class Adddata extends JFrame  {
  JPanel panel;            
  JLabel sumbook,sumfood,sum,time,l1;
  JTextField btxt,ftxt,stxt,ttxt;   
  JButton bacbtn, savebtn, resetbtn;
  Font fn = new Font("Angsana New", Font.BOLD, 30);
  Font fn1 = new Font("Angsana New", Font.BOLD, 20);
  AddPanel y;
  
  public Adddata() {
    setTitle("รับข้อมูลค่าใช้จ่าย");
    buildPanel();
    add(panel);
    setVisible(true);
  } 
  private void buildPanel() {
    
    l1 = new JLabel("รับข้อมูลค่าใช้จ่าย",SwingConstants.CENTER);
    l1.setForeground(Color.BLUE);
    time = new JLabel("รับข้อมูลวันที่บันทึก");
    sumbook = new JLabel("ค่าหนังสือ");
    sumfood = new JLabel("ค่าอาหาร");
    sum = new JLabel("ค่าใช้จ่ายอื่นๆ");
    btxt = new JTextField(15);
    ftxt = new JTextField(15);
    stxt  = new JTextField(15);
    ttxt  = new JTextField(15);
    bacbtn = new JButton("ย้อนกลับ");
    savebtn = new JButton("บันทึก");
    resetbtn = new JButton("รีเซท");  
	    
    l1.setFont(fn);
    time.setFont(fn1);
    sumbook.setFont(fn1);
    sumfood.setFont(fn1);
    sum.setFont(fn1);
   
    bacbtn.setFont(fn1);
    savebtn.setFont(fn1);
    resetbtn.setFont(fn1);    
    panel = new JPanel();
    panel.setLayout (new GridBagLayout ());    
    y = new AddPanel();
    
    y.addItem(panel,l1,0,0,1,1,GridBagConstraints.CENTER);
     
    y.addItem(panel,time,0,1,1,1,GridBagConstraints.WEST);
    y.addItem(panel,ttxt,1,1,2,1,GridBagConstraints.EAST);
       
    y.addItem(panel,sumbook,0,2,1,1,GridBagConstraints.WEST);
    y.addItem(panel,btxt,1,2,2,1,GridBagConstraints.EAST);

    y.addItem(panel,sumfood,0,3,1,1,GridBagConstraints.WEST);
    y.addItem(panel,ftxt,1,3,2,1,GridBagConstraints.EAST);
    
    y.addItem(panel,sum,0,4,1,1,GridBagConstraints.WEST);
    y.addItem(panel,stxt,1,4,2,1,GridBagConstraints.EAST);

    y.addItem(panel,bacbtn,0,5,1,1,GridBagConstraints.WEST);
    y.addItem(panel,savebtn,1,5,1,1,GridBagConstraints.WEST);
    y.addItem(panel,resetbtn,2,5,1,1,GridBagConstraints.WEST);
    
    bacbtn.addActionListener(new ButtonListener());    
    resetbtn.addActionListener(new ButtonListener());
    savebtn.addActionListener(new ButtonListener());
    
  }  
   private class ButtonListener implements ActionListener  {
    @Override
    public void actionPerformed(ActionEvent e)    {
      if (e.getSource()==bacbtn) {
	dispose();
      }
      if (e.getSource()==savebtn) {
          
              String sid =ttxt.getText();
             String sb =btxt.getText();
             String sf =ftxt.getText();
             String ss =stxt.getText();
            File ofile = new File("d:\\data.txt");
            FileWriter f_write = null;
            BufferedWriter b_write = null;
            PrintWriter p_write = null;
            String line;
            
            
           try{
      f_write = new FileWriter(ofile);
      b_write = new BufferedWriter(f_write);
      p_write = new PrintWriter(b_write);
  
         p_write.println(sid+sb+sf+ss);
             
           }
         catch (IOException ee) {
            }
         finally {              
          p_write.close();
        }
           JOptionPane.showMessageDialog(null, "Save data completed");
      }

      if (e.getSource()==resetbtn) {
	ttxt.setText("");
        btxt.setText("");
        ftxt.setText("");
        stxt.setText("");
        ttxt.requestFocus();
      } 
   } 
  } 
}