random access file in java

Question

Inventory control (16 byte record)

Product ID code (int – 4 bytes)

Quantity in stock (int – 4 bytes)

Price (double – 8 bytes)

How to create fixed length random access file using the above lengths? I tried some examples online, i either get EOF exception or random address values when I try to access them...

Again I did try some examples and couldn't understand the concept very well... I'm trying a project with it and try to explore more on it...

Example data There might be holes in the data where No. in stock could be 23 == 023

      Quantity

ID. No. In Stock Price

------- -------- ------

1001 476 $28.35

1002 240 $32.56

1003 517 $51.27

1004 284 $23.75

1005 165 $32.25

THanks for the help

Answer 1

java.io.RandomAccessFile is the class you're looking for. Here's an example implementation (you'll probably want to write some unit tests, as I haven't :)

package test;

import java.io.IOException;
import java.io.RandomAccessFile;

public class Raf {
    private static class Record{
     private final double price;
     private final int id;
     private final int stock;

     public Record(int id, int stock, double price){
      this.id = id;
      this.stock = stock;
      this.price = price;
     }

     public void pack(int n, int offset, byte[] array){
      array[offset + 0] = (byte)(n & 0xff);
      array[offset + 1] = (byte)((n >> 8) & 0xff);
      array[offset + 2] = (byte)((n >> 16) & 0xff);
      array[offset + 3] = (byte)((n >> 24) & 0xff);
     }

     public void pack(double n, int offset, byte[] array){
      long bytes = Double.doubleToRawLongBits(n);
      pack((int) (bytes & 0xffffffff), offset, array);
      pack((int) ((bytes >> 32) & 0xffffffff), offset + 4, array);
     }

     public byte[] getBytes() {
      byte[] record = new byte[16];
      pack(id, 0, record);
      pack(stock, 4, record);
      pack(price, 8, record);
      return record;
     }
    }

    private static final int RECORD_SIZE = 16;
    private static final int N_RECORDS = 1024;

    /**
     * @param args
     * @throws IOException 
     */
    public static void main(String[] args) throws IOException {
     RandomAccessFile raf = new RandomAccessFile(args[0], "rw");
     try{
      raf.seek(RECORD_SIZE * N_RECORDS);

      raf.seek(0);

      raf.write(new Record(1001, 476, 28.35).getBytes());
      raf.write(new Record(1002, 240, 32.56).getBytes());
     } finally {
      raf.close();
     }
    }
}