我需要有关登录系统的帮助

Question

我编写了有关登录系统的代码。它具有用户和管理员。关于用户，他们可以更改密码。用户帐户已经在txt文件中，并且当用户更改密码时，必须将新密码写入该txt文件中，并将其他用户保留在txt文件中。问题是，当我单击“更改密码”时，使用用户名写一个新密码，但在txt文件中仍然具有要更改密码的用户的旧密码。

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.util.Scanner;
import java.io.File;
import java.io.FileWriter;

public class TEST4
        {
            public static Scanner S;
            public static Scanner H;
            private JFrame Frame;

            private JPanel Panel;

            private JButton Button;
            private JButton Button1;

            private JLabel Label1;
            private JLabel Label;
            private JLabel Label3;
            private JLabel Label4;
            JTextField txt = new JTextField("",50);
            JTextField txt1 = new JTextField("",50);

            public static void main(String[] args)
                {

                System.out.println("Loading...");

                new TEST4 ();
                }
            public TEST4 ()
                {
                Application();
                }
            public void Application()
                {
                Frame = new JFrame("Welcome");
                Frame.setVisible(true);

                Frame.setSize(400,400);
                Frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                Panel = new JPanel();
                //Panel.setBackground(Color.BLACK);
                Panel.setLayout(null);//Override auto arrange elements

                txt1.setBounds(80, 50, 100, 25);
                txt.setBounds(80, 10, 100, 25);

                Button = new JButton("Sign in");
                Button.setBounds(80, 90, 100, 25);//(x,y,w,h)
                Label = new JLabel("username : ");
                Label.setBounds(10, 10, 100, 25);
                Label1 = new JLabel("password : ");
                Label1.setBounds(10, 50, 100, 25);

                Panel.add(Button);
                Panel.add(Label);
                Panel.add(Label1);

                Panel.add(txt);
                Panel.add(txt1);
                Frame.add(Panel);
                Button.addActionListener(new ActionListener()
                    {
                    public void actionPerformed(ActionEvent e)
                        {
                        String X=txt.getText();
                        String X1=txt1.getText();
                        String filepath="username.txt";

                        Search(X,X1,filepath);  
                        Frame.setVisible(false);            
                                JFrame Frame2 = new JFrame("User Setting");
                                Frame2.setVisible(true);

                                Frame2.setSize(400,400);
                                Frame2.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                                JPanel Panel2 = new JPanel(); 
                                Panel2.setLayout(null);
                                Label3= new JLabel("Welcome user");
                                Label3.setBounds(5, 10, 100, 25);
                                JTextField txt3 = new JTextField("",50);
                                txt3.setBounds(80,50,100,25);
                                Button1= new JButton("Change password");
                                Button1.setBounds(180, 60, 100, 25);
                                Button1.addActionListener(new ActionListener()
                                    {
                                    public void actionPerformed(ActionEvent b)
                                    {
                                    String X2=txt3.getText();
                                    String user="";
                                    String pass="";
                                    boolean found=false;

                                    try
                                    {
                                    H= new Scanner(new File(filepath));
                                    H.useDelimiter("[,\n]");

                                    while (H.hasNext() && !found)
                                        {
                                            user=H.next();
                                            pass=H.next();
                                            FileWriter H=new FileWriter("username.txt",true);

                                            H.close();
                                        if(user.trim().equals(X.trim()) && pass.trim().equals(X1.trim()))
                                            {
                                            FileWriter J = new FileWriter("username.txt",true);
                                            J.write("\r\n");
                                            J.write(X+","+X2);
                                            J.write("\r\n");




                                            J.close();
                                            }
                                        }
                                        H.close();


                                    }
                                    catch(Exception h)
                                    {
                                    System.out .print("error");                         
                                    }
                                    }
                                    });
                                    Panel2.add(Label3);
                                    Panel2.add(txt3);
                                    Panel2.add(Button1);    
                                    Frame2.add(Panel2);     
                                if(X.equals("tan123") && X1.equals("123456"))
                            {
                                Frame.setVisible(false);
                                Frame2.setVisible(false);
                                JFrame Frame3= new JFrame("Admin setting");
                                Frame3.setVisible(true);
                                Frame3.setSize(400,400);
                                Frame3.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                                JPanel Panel3 = new JPanel(); 
                                Panel3.setLayout(null);
                                Label4= new JLabel("Welcome admin");
                                Label4.setBounds(5, 10, 100, 25);
                                JTextField txt4 = new JTextField("",50);
                                txt4.setBounds(80,50,100,25);
                                    Panel3.add(Label4);
                                    Panel3.add(txt4);

                                    Frame3.add(Panel3);
                            }


                        }   




                        });

                }
            public static void Search(String X,String X1,String filepath)
                {
                    boolean found=false;
                    String username="";
                    String password="";
                try
                {

                    S=new Scanner (new File(filepath));
                    S.useDelimiter("[,\n]");

                    while (S.hasNext() && !found)
                    {
                        username=S.next();
                        password=S.next();
                    if(username.trim().equals(X.trim()) && password.trim().equals(X1.trim()))
                        {
                        found=true;
                        }

                    }

                    S.close();

                    if(found==true)
                        {
                        System.out.print("welcome");
                        }

                    else
                        {
                        System.out.print("invalid");
                        }
                }
                catch(Exception a)
                {
                    System.out.println("error");
                }


    }
    }

为此一个新人，所以还是一团糟。

这是具有旧用户名和密码的旧txt文件

tan,123

shay,098

当我将用户tan的密码更改为456时，就这样

tan,123

shay,098tan,456

如何删除tan,123并将其替换为tan,456并仍然保留shay,098。 对不起，我的英语不好，谢谢！！！