Randomaccessfile, writing an update to the file?

Question

Hi,
I'm working on an app that uses a randomaccessfile (student.dat) and I have it so it will write info to the dat file & I can search thru what I entered..... now I need to add the ability to update a record in the dat. The student info is just basic name & address. So I need to be able to update an address change for a given record.
I looked at the documentation online but I'm not understanding how you capture the reference point in the file and modify it.... does anyone have a good example or does anyone out there know how to explain how to do it to me??
My file is here below that I need to add the update functionality to (and the 2 accompaning files)...... I already inserted my Update button on the second panel where it needs to reside. Now I need to add the update functionality to the button.
Anyone have any suggestions/explanations on how to add this fnctionality or know a working example I can reference?? Any help would be great appreciated.... thanks.
// StudentRecords.java: Store and read data
// using RandomAccessFile
import java.io.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
public class StudentRecords extends JFrame {
  // Create a tabbed pane to hold two panels
  private JTabbedPane jtpStudent = new JTabbedPane();
  // Random access file for access the student.dat file
  private RandomAccessFile raf;
  /** Main method */
  public static void main(String[] args) {
    StudentRecords frame = new StudentRecords();
    frame.pack();
    frame.setTitle("Test RandomAccessFile");
    frame.setVisible(true);
  /** Default constructor */
  public StudentRecords() {
    // Open or create a random access file
    try {
      raf = new RandomAccessFile("student.dat", "rw");
    catch(IOException ex) {
      System.out.print("Error: " + ex);
      System.exit(0);
    // Place buttons in the tabbed pane
    jtpStudent.add(new RegisterStudent(raf), "Register Student");
    jtpStudent.add(new ViewStudent(raf), "View Student");
    // Add the tabbed pane to the frame
    getContentPane().add(jtpStudent);
// Register student panel
class RegisterStudent extends JPanel implements ActionListener {
  // Button for registering a student
  private JButton jbtRegister;
  // Student information panel
  private StudentPanel studentPanel;
  // Random access file
  private RandomAccessFile raf;
  public RegisterStudent(RandomAccessFile raf) {
    // Pass raf to RegisterStudent Panel
    this.raf = raf;
    // Add studentPanel and jbtRegister in the panel
    setLayout(new BorderLayout());
    add(studentPanel = new StudentPanel(),
      BorderLayout.CENTER);
    add(jbtRegister = new JButton("Register"),
      BorderLayout.SOUTH);
    // Register listener
    jbtRegister.addActionListener(this);
  /** Handle button actions */
//   JFileChooser fc;
  public void actionPerformed(ActionEvent e) {
    if (e.getSource() == jbtRegister) {
      Student student = studentPanel.getStudent();
      try {
        raf.seek(raf.length());
        student.writeStudent(raf);
      catch(IOException ex) {
        System.out.print("Error: " + ex);
// View student panel
class ViewStudent extends JPanel implements ActionListener {
  // Buttons for viewing student information
  private JButton jbtFirst, jbtNext, jbtPrevious, jbtLast, jbtUpdate;
  // Random access file
  private RandomAccessFile raf = null;
  // Current student record
  private Student student = new Student();
  // Create a student panel
  private StudentPanel studentPanel = new StudentPanel();
  // File pointer in the random access file
  private long lastPos;
  private long currentPos;
  public ViewStudent(RandomAccessFile raf) {
    // Pass raf to ViewStudent
    this.raf = raf;
    // Panel p to hold four navigator buttons
    JPanel p = new JPanel();
    p.setLayout(new FlowLayout(FlowLayout.LEFT));
    p.add(jbtFirst = new JButton("First"));
    p.add(jbtNext = new JButton("Next"));
    p.add(jbtPrevious = new JButton("Previous"));
    p.add(jbtLast = new JButton("Last"));
    p.add(jbtUpdate = new JButton("Update"));
    // Add panel p and studentPanel to ViewPanel
    setLayout(new BorderLayout());
    add(studentPanel, BorderLayout.CENTER);
    add(p, BorderLayout.SOUTH);
    // Register listeners
    jbtFirst.addActionListener(this);
    jbtNext.addActionListener(this);
    jbtPrevious.addActionListener(this);
    jbtLast.addActionListener(this);
    jbtUpdate.addActionListener(this);
  /** Handle navigation button actions */
  public void actionPerformed(ActionEvent e) {
    String actionCommand = e.getActionCommand();
    if (e.getSource() instanceof JButton) {
      try {
        if ("First".equals(actionCommand)) {
          if (raf.length() > 0)
            retrieve(0);
        else if ("Next".equals(actionCommand)) {
          currentPos = raf.getFilePointer();
          if (currentPos < raf.length())
            retrieve(currentPos);
        else if ("Previous".equals(actionCommand)) {
          currentPos = raf.getFilePointer();
          if (currentPos > 0)
            retrieve(currentPos - 2*2*Student.RECORD_SIZE);
        else if ("Last".equals(actionCommand)) {
          lastPos = raf.length();
          if (lastPos > 0)
            retrieve(lastPos - 2*Student.RECORD_SIZE);
        else if ("Update".equals(actionCommand)) {
     //     raf.writeChars(student.dat);     //HOW DO I UPDATE THE FILE????????????????
      catch(IOException ex) {
        System.out.print("Error: " + ex);
  /** Retrieve a record at specified position */
  public void retrieve(long pos) {
    try {
      raf.seek(pos);
      student.readStudent(raf);
      studentPanel.setStudent(student);
    catch(IOException ex) {
      System.out.print("Error: " + ex);
// This class contains static methods for reading and writing
// fixed length records
class FixedLengthStringIO {
  // Read fixed number of characters from a DataInput stream
  public static String readFixedLengthString(int size,
    DataInput in) throws IOException {
    char c[] = new char[size];
    for (int i=0; i<size; i++)
      c[i] = in.readChar();
    return new String(c);
  // Write fixed number of characters (string s with padded spaces)
  // to a DataOutput stream
  public static void writeFixedLengthString(String s, int size,
    DataOutput out) throws IOException {
    char cBuffer[] = new char[size];
    s.getChars(0, s.length(), cBuffer, 0);
    for (int i = s.length(); i < cBuffer.length; i++)
      cBuffer[i] = ' ';
    String newS = new String(cBuffer);
    out.writeChars(newS);
}Student Panel....................
// StudentPanel.java: Panel for displaying student information
import javax.swing.*;
import javax.swing.border.*;
import java.awt.*;
public class StudentPanel extends JPanel {
  JTextField jtfName = new JTextField(32);
  JTextField jtfStreet = new JTextField(32);
  JTextField jtfCity = new JTextField(20);
  JTextField jtfState = new JTextField(2);
  JTextField jtfZip = new JTextField(5);
  /** Construct a student panel */
  public StudentPanel() {
    // Set the panel with line border
    setBorder(new BevelBorder(BevelBorder.RAISED));
    // Panel p1 for holding labels Name, Street, and City
    JPanel p1 = new JPanel();
    p1.setLayout(new GridLayout(3, 1));
    p1.add(new JLabel("Name"));
    p1.add(new JLabel("Street"));
    p1.add(new JLabel("City"));
    // Panel jpState for holding state
    JPanel jpState = new JPanel();
    jpState.setLayout(new BorderLayout());
    jpState.add(new JLabel("State"), BorderLayout.WEST);
    jpState.add(jtfState, BorderLayout.CENTER);
    // Panel jpZip for holding zip
    JPanel jpZip = new JPanel();
    jpZip.setLayout(new BorderLayout());
    jpZip.add(new JLabel("Zip"), BorderLayout.WEST);
    jpZip.add(jtfZip, BorderLayout.CENTER);
    // Panel p2 for holding jpState and jpZip
    JPanel p2 = new JPanel();
    p2.setLayout(new BorderLayout());
    p2.add(jpState, BorderLayout.WEST);
    p2.add(jpZip, BorderLayout.CENTER);
    // Panel p3 for holding jtfCity and p2
    JPanel p3 = new JPanel();
    p3.setLayout(new BorderLayout());
    p3.add(jtfCity, BorderLayout.CENTER);
    p3.add(p2, BorderLayout.EAST);
    // Panel p4 for holding jtfName, jtfStreet, and p3
    JPanel p4 = new JPanel();
    p4.setLayout(new GridLayout(3, 1));
    p4.add(jtfName);
    p4.add(jtfStreet);
    p4.add(p3);
    // Place p1 and p4 into StudentPanel
    setLayout(new BorderLayout());
    add(p1, BorderLayout.WEST);
    add(p4, BorderLayout.CENTER);
  /** Get student information from the text fields */
  public Student getStudent() {
    return new Student(jtfName.getText().trim(),
                       jtfStreet.getText().trim(),
                       jtfCity.getText().trim(),
                       jtfState.getText().trim(),
                       jtfZip.getText().trim());
  /** Set student information on the text fields */
  public void setStudent(Student s) {
    jtfName.setText(s.getName());
    jtfStreet.setText(s.getStreet());
    jtfCity.setText(s.getCity());
    jtfState.setText(s.getState());
    jtfZip.setText(s.getZip());
}Student file............
// Student.java: Student class encapsulates student information
import java.io.*;
public class Student implements Serializable {
  private String name;
  private String street;
  private String city;
  private String state;
  private String zip;
  // Specify the size of five string fields in the record
  final static int NAME_SIZE = 32;
  final static int STREET_SIZE = 32;
  final static int CITY_SIZE = 20;
  final static int STATE_SIZE = 2;
  final static int ZIP_SIZE = 5;
  // the total size of the record in bytes, a Unicode
  // character is 2 bytes size
  final static int RECORD_SIZE =
    (NAME_SIZE + STREET_SIZE + CITY_SIZE + STATE_SIZE + ZIP_SIZE);
  /** Default constructor */
  public Student() {
  /** Construct a Student with specified name, street, city, state,
     and zip
  public Student(String name, String street, String city,
    String state, String zip) {
    this.name = name;
    this.street = street;
    this.city = city;
    this.state = state;
    this.zip = zip;
  /** Return name */
  public String getName() {
    return name;
  /** Return street */
  public String getStreet() {
    return street;
  /** Return city */
  public String getCity() {
    return city;
  /** Return state */
  public String getState() {
    return state;
  /** Return zip */
  public String getZip() {
    return zip;
  /** Write a student to a data output stream */
  public void writeStudent(DataOutput out) throws IOException {
    FixedLengthStringIO.writeFixedLengthString(
      name, NAME_SIZE, out);
    FixedLengthStringIO.writeFixedLengthString(
      street, STREET_SIZE, out);
    FixedLengthStringIO.writeFixedLengthString(
      city, CITY_SIZE, out);
    FixedLengthStringIO.writeFixedLengthString(
      state, STATE_SIZE, out);
    FixedLengthStringIO.writeFixedLengthString(
      zip, ZIP_SIZE, out);
  /** Read a student from data input stream */
  public void readStudent(DataInput in) throws IOException {
    name = FixedLengthStringIO.readFixedLengthString(
      NAME_SIZE, in);
    street = FixedLengthStringIO.readFixedLengthString(
      STREET_SIZE, in);
    city = FixedLengthStringIO.readFixedLengthString(
      CITY_SIZE, in);
    state = FixedLengthStringIO.readFixedLengthString(
      STATE_SIZE, in);
    zip = FixedLengthStringIO.readFixedLengthString(
      ZIP_SIZE, in);

Answer 1

Advise sounds strikingly similar to what I mentioned
page b4. Look, can you post the entire class in code
tags ... and the entire file as well?@Bill,
Yes, I read yours but (sorry, its hard to answer multiple replies in these forums) but I could not understand some of what you suggested. If you look at the //???? lines I commented on your code below you'll see where I'm lost on what you are doing there. You also mentioned a "marker"... is that the currentPos thing in the code of mine, to get the current position?
Both you and Andrew suggested classes to some extent but I'm not sure how to it or what goes in it. If you look at my code paste above the buttons seem to have everything in the action, not a class, other than the register button on the other pane (which has a write method in studentpanel.java).
I'm trying to follow/implement what u guys have been suggesting... but my brain just is not wrapping around the what I should type & where..... :-( Got any further explanation/demonstration of what I should be doing?? Sorry, I'm not that skilled of a programmer yet...
int curr_ptr = 0,
prev_ptr = 0;
String marker = "ADDRS=",
address="My new address",
record = null;
while ( ( record = raf.readLine() ) != null ) {   //????
curr_ptr = raf.getFilePointer();
if ( ( idx = record.indexOf("marker") ) != -1 ) {   //????
record = record.substring( 0, 6 ) + address;  //????
raf.seek( prev_ptr );
raf.writeBytes( record );
curr_ptr = raf.getFilePointer();
raf.seek(curr_ptr);
}