Read a file and maintain a file pointer?

Question

Hi,
I need to read a file which has about 1000 records(approx).I need to read 10 records from the beginning of the file and then give the 10 records to a different java process and then continue extracting records for every 10.So how do I save the filepointer and then continue read from the 11th record and then 21st record and so on?
Remember I don't know the total number of records in the file,but know where exactly I have to extract,I mean the trailer info.
also, Is there a way to store a file pointer for a particular position in the file and then continue reading the file from the same place by using the saved filepointer?
any idea?
also,If I save a filepointer as long value and open the file later using randomaccess file,and seek using this saved long value,will there be any change in the position of the file?will this differ in unix and windows?I mean the file pointer?
--Robert

Answer 1

Maybe this will make things clearer. Don't be intimidated, it's not really hardimport java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.Random;
import java.io.*;
public class Test extends JFrame {
    static final long RECORD_SIZE = 8;
    RandomAccessFile rf;
    JTextArea jta = new JTextArea();
    JTextField jtf = new JTextField();
    Random r = new Random();
    public Test() {
     try { rf = new RandomAccessFile("rtest.dat", "rw"); }
     catch (FileNotFoundException fnfe) { fnfe.printStackTrace(); System.exit(1); }
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     Container content = getContentPane();
     // Set up the GUI
     content.add(new JScrollPane(jta), BorderLayout.CENTER);
     jtf.setPreferredSize(new Dimension(100,20));
     content.add(jtf, BorderLayout.NORTH);
     JPanel buttonPanel = new JPanel();
     content.add(buttonPanel, BorderLayout.SOUTH);
     JButton add10 = new JButton("Add 10");
     buttonPanel.add(add10);
     add10.addActionListener(new ActionListener() {   // Add 10 listener
         public void actionPerformed(ActionEvent ae) {
          try {
              rf.seek(rf.length());                // seek to end
              for(int i = 0; i < 10; i++)          // write 10 recs
                   rf.writeDouble(r.nextDouble()*1000000.);
          catch (IOException ioe) { ioe.printStackTrace(); }
          jta.setText("10 records appended");
          showStats();                        // show current statistics
     JButton list10 = new JButton("List Next 10");
     buttonPanel.add(list10);
     list10.addActionListener(new ActionListener() {    // list 10 listener
         public void actionPerformed(ActionEvent ae) {
          jta.setText(null);
          try {
              for(int i = 0; i < 10; i++) {
               if (rf.getFilePointer()<rf.length()) {  // if not eof
                   long recNo = rf.getFilePointer()/RECORD_SIZE;
                   jta.append(""+recNo+") "+Double.toString(rf.readDouble())+"\n");
               } else jta.append("(eof)\n");
          } catch (IOException ioe) { ioe.printStackTrace(); }
          showStats();                        // show current location/size
     JButton random10 = new JButton("List Random 10");
     buttonPanel.add(random10);
     random10.addActionListener(new ActionListener() {   // list 10 random records
         public void actionPerformed(ActionEvent ae) {
          jta.setText(null);
          try {
              for(int i = 0; i < 10; i++) {
               // Get a rendom record number
               int recNo = r.nextInt((int)(rf.getFilePointer()/RECORD_SIZE));
               // seek to it
               rf.seek(recNo*RECORD_SIZE);
               // read record, add to textarea
               jta.append(""+recNo+") "+Double.toString(rf.readDouble())+"\n");
          } catch (IOException ioe) { ioe.printStackTrace(); }
          showStats();
     buttonPanel.add(random10);
     JButton rewind = new JButton("Rewind");
     buttonPanel.add(rewind);
     rewind.addActionListener(new ActionListener() {  // rewind listener
         public void actionPerformed(ActionEvent ae) {
          jta.setText(null);
          try {
              rf.seek(0);                     // move to beginning
          } catch (IOException ioe) { ioe.printStackTrace(); }
          jta.setText("Rewound");
          showStats();                    // show size/location
     buttonPanel.add(rewind);
     showStats();
     setSize(600,500);
     show();
    private void showStats() {
     try {
         jtf.setText("Cnt="+(rf.length()/RECORD_SIZE)+
               ", Cur="+(rf.getFilePointer()/RECORD_SIZE));
     } catch (IOException ioe) { ioe.printStackTrace(); }
    public static void main(String[] args) { new Test(); }
}