Re: Random Access File IO

From:

"Charles Hottel" <chottel@earthlink.net>

Newsgroups:

comp.lang.java.help

Date:

Sat, 11 Nov 2006 03:35:49 GMT

Message-ID:

<pyb5h.5054$l25.4737@newsread4.news.pas.earthlink.net>

"GreenMountainBoy" <william.fiske@gmail.com> wrote in message
news:1163173422.582728.173160@h54g2000cwb.googlegroups.com...

HI,

I really would appreciateit if someone could provide me with some
advice (sample code) on how to write a simple random access file I/O
program in java.

the record structure is

public employee {

int iRecordNumber;

String sName;

double dSalary;

}

HOW DO I CREATE A SIMPLE PROGRAM TO DO RANDOM ACCESS FILE I/O?

I need to be able to create these records from the keyboard, and then
write them to a random access file using the iRecordNumber as the key.
Part of the problem is that the sName field should be fixede at 25
characters in length, but I dont know how to do that.

Then, I need to be able to read back in a record by entering its
iRecordNumber from the keyboard,

and ideally, should be able to delete a random record.

Thanks for your help - any assistance would be very much appreciated!

GreenMountainBoy

Here is an example froma Mike Murach book:

import java.io.*;
import java.util.*;

public class BookIO{
private static Book book = null;
private static String[] codes = null;
private static RandomAccessFile randomFile = null;

private static final File BOOK_FILE = new File("books.dat");
private static final int CODE_SIZE = 4;
private static final int TITLE_SIZE = 20;
private static final int RECORD_SIZE = CODE_SIZE*2 + TITLE_SIZE*2 + 8;

public static void open() throws IOException{
  randomFile = new RandomAccessFile(BOOK_FILE, "rw");
  codes = readCodes();
}

public static void close(){
  try{
   randomFile.close();
  }
  catch(IOException e){
   System.out.println("I/OException thrown when closing file.");
  }
}

public static int getRecordCount() throws IOException{
  long length = BOOK_FILE.length();
  int recordCount = (int) (length / RECORD_SIZE);
  return recordCount;
}

public static String readString(DataInput in, int length)
throws IOException{
  String s = "";
  int i = 0;
  while (i < length){
   char c = in.readChar();
   if (c!=0)
    s += c;
   i++;
  }
  return s;
}

public static Book readRecord(int recordNumber) throws IOException{
  randomFile.seek((recordNumber-1) * RECORD_SIZE);
  String code = readString(randomFile, CODE_SIZE);
  String title = readString(randomFile, TITLE_SIZE);
  double price = randomFile.readDouble();
  book = new Book(code, title, price);
  return book;
}

public static Book readRecord(String bookCode) throws IOException{
  int recordNumber = getRecordNumber(bookCode);
  book = readRecord(recordNumber);
  return book;
}

public static int getRecordNumber(String bookCode) throws IOException{
  int match = -1;
  int i = 0;
  boolean flag = true;
  while ((i < getRecordCount()) && (flag==true)){
   if (bookCode.equals(codes[i])){
    match = i+1;
    flag = false;
   }
   i++;
  }
  return match;
}

public static String[] readCodes() throws IOException{
  codes = new String[getRecordCount()];
  for (int i = 0; i < getRecordCount(); i++){
   randomFile.seek(i * RECORD_SIZE);
   codes[i] = readString(randomFile, CODE_SIZE);
  }
  return codes;
}

public static String[] readTitles() throws IOException{
  String[] titles = new String[getRecordCount()];
  for (int i = 0; i < getRecordCount(); i++){
   randomFile.seek(i * RECORD_SIZE + 8);
   titles[i] = readString(randomFile, TITLE_SIZE);
  }
  return titles;
}

public static void writeString(DataOutput out, String s, int length)
throws IOException{
  for (int i = 0; i < length; i++){
   if (i < s.length())
    out.writeChar(s.charAt(i));
   else
    out.writeChar(0);
  }
}

public static void writeRecord(Book book, int recordNumber)
throws IOException{
  randomFile.seek((recordNumber-1) * RECORD_SIZE);
  writeString(randomFile, book.getCode(), CODE_SIZE);
  writeString(randomFile, book.getTitle(), TITLE_SIZE);
  randomFile.writeDouble(book.getPrice());
}

public static Book moveFirst() throws IOException{
  book = readRecord(1);
  return book;
}

public static Book movePrevious() throws IOException{
  //add code to handle if Prev button clicked on first record.
  int recordNumber = getRecordNumber(book.getCode());
  book = readRecord(recordNumber - 1);
  return book;
}

public static Book moveNext() throws IOException{
  //add code to handle if Next button clicked on last record.
  int recordNumber = getRecordNumber(book.getCode());
  book = readRecord(recordNumber + 1);
  return book;
}

public static Book moveLast() throws IOException{
  int lastRecordNumber = getRecordCount();
  book = readRecord(lastRecordNumber);
  return book;
}

public static void addRecord(Book addBook) throws IOException{
  writeRecord(addBook, getRecordCount() + 1);
  close();
  open();
}

public static void updateRecord(Book book) throws IOException{
  int recordNumber = getRecordNumber(book.getCode());
  //record needs to be written to file
}

public static void deleteRecord(String bookCode) throws IOException{
  int recordNumber = getRecordNumber(bookCode);
  Vector books = new Vector();
  for (int i = 0; i< getRecordCount(); i++){
   books.add(readRecord(i+1));
  }
  books.remove(recordNumber-1);
  randomFile.setLength(RECORD_SIZE *(getRecordCount() -1));
  for (int i = 0; i<books.size(); i++){
   writeRecord((Book)books.elementAt(i),i+1);
  }
  if (recordNumber < getRecordCount())
   book = readRecord(recordNumber);
  else
   book = readRecord(getRecordCount()-1);
  close();
  open();
}
}