Re: Sample code for random access file io program

From:
Tom Forsmo <spam@nospam.net>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 16 Nov 2006 12:55:19 +0100
Message-ID:
<455c5119$1@news.broadpark.no>
GreenMountainBoy wrote:

The record structure is

public Employee

{
    int iRecordNumber;
    String sName;
    double dSalary;
}


First a quick question, are you absolutely sure it needs to be record
based? Could it not be textbased? this is much easier in java than
records E.g.

recordnum=1024
sname=xxxxxx xxxxxxxx
salary=3.14

In any case, what you probably should do is use a byte array, set up for
the exact size of the record, then for each employee oject you add a
method that returns a byte array representation of the object. then you
use a list iterator to iterate through the list and give you each byte
array representation in the list.

e.g. record format in BigEndian format, with \0 based padding

   num (4 bytes) | sname (25 bytes) | salary (8 bytes)

    public interface Recordformat {
        public byte[] toRecord();
    }

    public class Employee implements RecordFormat {
    ...
        public byte[] toRecord() {
    }

I need to be able to create these records from the keyboard, then write
them to disk, and then be able to read one based on a iRecordNumber.
Part of the problem is that the Name string is variable length and I
need it to be a fixed 25 characters - I have found that character
arrays won't function in many places where a string is called for..


Why is that?

I need to be able to 1) add new items, 2) list all items, 3) query an
item based on its record number, 4) update a record (salary only), 5)
delete an item.


When it comes to the file access you could use java.io.RandomAccessFile
to make a filemanager object which traverses the file in record size
chunks, both backwards and forwards, and allows you to save both single
records and a list of records. You can delete a record by \0 padding it,
or just rewrite the entire file with the updated list.

tom

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