Random access file in java

Question

I have the following fields:

• Inventory management (16 byte write)
  • Product Identifier Code (int - 4 bytes)
  • Quantity in stock (int - 4 bytes)
  • Price (double - 8 bytes)

How to create a fixed-length random access file using the above lengths? I tried several examples on the Internet, but when I try to access them, I either get an EOF exception or random values.

I tried a few more examples and could not understand the concept very well. I am trying to create a project with it and will try to learn more about it.

Here are some sample data. There may be holes in the data where No. in stock No. in stock may 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 are looking for. Here is an implementation example (you probably want to write some unit tests, since I don't have them :)

 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(); } } }