Question

I have the following fields:

• Inventory control (16 byte record)
  • Product ID code (int – 4 bytes)
  • Quantity in stock (int – 4 bytes)
  • Price (double – 8 bytes)

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

I tried some more examples and couldn’t understand the concept very well. I’m trying a project with it and will try to explore more on it.

Here is some 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();         }     } }