How can i delete a UserName from a text file using Strings or io.

Question

hi i m trainee
i have been assigned to make java program which deletes a UserName and
his Passwor from a Text File
i m unable to do it using the code below
plz help
do reply
  import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java .applet.*;
import java.io.*;
class Del extends JFrame  implements ActionListener
     String s2;
     JTextField CWDMUserDText = new JTextField();
     JButton CWDMSecDelButton = new JButton("DELETE");
     JLabel CWDMUserLabel = new JLabel("USER NAME");
     JPasswordField CWDMPassDText = new JPasswordField();
     JLabel CWDMPassLabel = new JLabel("PASSWORD");
     //String user,pass;
     /* CONSTRUCTOR*/
          Del()
               Container contentPane=getContentPane();
               contentPane.setLayout(null);
              setLocation(400,200);
               contentPane.add(CWDMUserDText);
               contentPane.add(CWDMSecDelButton);
               contentPane.add(CWDMUserLabel);
               contentPane.add(CWDMPassDText);
               contentPane.add(CWDMPassLabel);
              CWDMUserLabel.setBounds(20,70,100,25);
              CWDMPassLabel.setBounds(20,100,100,25);
               CWDMUserDText.setBounds(150,70,100,25);
               CWDMPassDText.setBounds(150,100,100,25);
               CWDMSecDelButton.setBounds(80,135,100,25);
               CWDMUserDText.addActionListener(this);
               CWDMPassDText.addActionListener(this);
               CWDMSecDelButton.addActionListener(this);
               contentPane.setBackground(Color.CYAN);
               setVisible(true);
               setDefaultCloseOperation(EXIT_ON_CLOSE);
     public void actionPerformed(ActionEvent ae)
              if(ae.getSource()==CWDMSecDelButton)
     //             user = CWDMUserDText.getText();
     //          pass = CWDMPassDText.getText();
     //          showarr();
               s2 = CWDMUserDText.getText();
               CWDMUserDText.setText("");
               CWDMPassDText.setText("");
               DelUser(s2);
          public static void main(String args[])
               Del myframe1= new Del();
               myframe1.setSize(300,200);
               myframe1.setVisible(true);
          public void DelUser(String s)
               //int a;
               try
                    FileReader fr = new FileReader("file.txt");
                    BufferedReader br = new BufferedReader(fr);
                   String s1;
                   int a;
                   int len = s.length();
                   System.out.println(len);
                   //s2 = CWDMUserDText.getText();
                    while ((s1=br.readLine()) !=null)
                         if((a=s1.indexOf(s))>0 && (a=s1.indexOf(s))!=0)
                              System.out.println(a);
                    fr.close();
               catch(FileNotFoundException e)
                    System.out.println("exception occured");
               catch(IOException e)
                    System.out.println("io");
     }

Answer 1

Some tips:
1) If you are adding or deleting stuff from a text file you need to write a new, modified file and then, optionally, do something like:
rename the old file to whatever.bak or the like.
rename the new file to the old.
2) Don't depend on System.out so much on a GUI application. For example if you get an exception use javax.swing.JOptionPane to display an alert (and show the message from the exception). You can write the stack trace to System.out if you want.
3) Don't muck about with calculating bounds for screen objects, let a layout manager sort it out. I like BoxLayoutManager for most things like this.
4) For bonus points do your IO in a separate Thread. Generally you don't want anything happening in an actionPerformed method which significantly delays it's return. The method should launch a new Thread to do the job, and disable the button until the Thread finishes.