NX: Dynamic DB

[Chris Harris]

Hi all,
I am trying to make my DB as dynamic as possible; using the header and field data within the database.
I have created a class called DBMetaData that holds all the header and field data. Within my Data class I have created ByteBuffer recordData[] that holds a single record. See code below:

  public Data(DBMetaData metaData) throws IOException {
    this.metaData = metaData;
    dbChannel = new RandomAccessFile(metaData.getDBName(),"rw").getChannel();
    
    recordData = new ByteBuffer[metaData.getNumOfFields()];
    int [] fieldlength = metaData.getAllFieldLength();
    for (int i = 0; i <  fieldlength.length; i ++) {
      recordData [i] = ByteBuffer.allocate(fieldlength[i]);
    } 
  }  

Has any one else noticed that the order of the fields description within the database is not the name as the order of the records.
I wanted my read method to look something like:

  public String[] read(int recNo) throws RecordNotFoundException {
                             
    try {
      long recordSize = dbChannel.size() - 70;
      long numRecord = recordSize / 184;
     
      
      dbChannel.position(72 + (184 * recNo));
      dbChannel.read(recordData);
      //Sort out return
    } catch (IOException e) {
    // TODO 
      e.printStackTrace();
    }
    // TODO
    return null;
  }

However this will not work, beacuse of the order that the records fields have been written.
So my question can anyone think of a way of sorting the fields data into the same order as the records and still keeping my code dynamic?
I hope you can understand what I mean.
Thanks
Chris
[ September 23, 2003: Message edited by: Chris Harris ]
[Andrew: Put code into [code] blocks]
[ September 23, 2003: Message edited by: Andrew Monkhouse ]

[Chris Harris]

Sorry for all the code blocks. I am still getting used to this forum.

[Philippe Maquet]

Hi Chris,
Do you mean that fields order as given in the header section of your file does not match fields order within the records ?!
Which assignment (including version number) have you ?
Regards,
Phil.

[Chris Harris]

I have the Bodgitt and Scarper assigment. After opening the DB in TextPad it looks like the field description are in the same order as in the records. SO there must be something wrong with the way i am storing the field names. hmm I will have to look in to it
Should I allow the client to get the field names from the DB so that they can be used in the JTable?
Thanks

[Philippe Maquet]

Hi Chris,

SO there must be something wrong with the way i am storing the field names. hmm I will have to look in to it

Indeed.

Should I allow the client to get the field names from the DB so that they can be used in the JTable?

It sounds like a good idea.
Good luck with your assignment.
Best,
Phil.

[Chris Harris]

Hi,
The assigment stats that:

Any unimplemented exceptions in this interface must all be created as member classes of the suncertify.db package. Each must have a zero argument constructor and a second constructor that takes a String that serves as the exception's description.

Do you think I will lose marks if I add another constructor that takes a Throwable cause?

[Philippe Maquet]

Hi Chris,

Do you think I will lose marks if I add another constructor that takes a Throwable cause?

No, IMO. I interpret the instructions as this : the zero argument constructor and the one that takes a String must be there as a minimum.
Best,
Phil.

[Chris Harris]

Hi Phil,
Thanks, as my plan is to wrap the IOException that could be thrown when attemting to read the DB into the RecordNotFoundException.
Chris

[Andrew Monkhouse]

Hi Chris,
I have put all your code between [code] and [/code] tags. If you edit your orignal post you will see where I put them. They help to preserve the indenting of your code which makes it easier to read. There is a button below the edit pane which will insert the [code] and [/code] tags into your post so you don't have to try and remember how to do it.
It seems like Philippe has already answered your questions
Regarding the code

      long recordSize = dbChannel.size() - 70;
      long numRecord = recordSize / 184;

I think "recordSize" is really "dataSectionSize".
I would recommend against using hard coded values for schema size and record length. I think they should either be constants or calculated from the file you are reading (my preference would be the later).
Regards, Andrew

[Chris Harris]

Thanks for the advice. I am about to start on the find method and have a few question. If you think I should start a new topic for this please say. The assigment states:

 
// Returns an array of record numbers that match the specified
// criteria. Field n in the database file is described by
// criteria[n]. A null value in criteria[n] matches any field
// value. A non-null  value in criteria[n] matches any field
// value that begins with criteria[n]. (For example, "Fred"
// matches "Fred" or "Freddy".)
public int[] find(String[] criteria);
 

What does this really mean?
1) would "Fred" match a record of "Tom, Fred"? i.e should the input be compared against all tokens in the record?
2) would the input "Tom Fred" match the record "Fred"? i.e should each tokens in the input be compare against the each token of the record?
My current idea for this problem involves using the new split method in String with the new regular expression classes. Or would I be better to uses a StringTokenizer instead of the split method?

Chris

[Andrew Monkhouse]

Hi Chris,
I think you should open a new topic for this - it is not really part of the "dynamic db" discussions.

1) would "Fred" match a record of "Tom, Fred"? i.e should the input be compared against all tokens in the record?

No, the starting characters do not match.

2) would the input "Tom Fred" match the record "Fred"? i.e should each tokens in the input be compare against the each token of the record?

No, the starting characters do not match.
All you are looking for anything that starts with the search parameter. You are not looking for a "contains" or "ends with".
Note that the output from the find will give more data than you want to display in your GUI, which should be limited to exact matches. (check this in your instructions though - I could be wrong).
Regards, Andrew

[Mogens Nidding]

Chris Harris: Do you think I will lose marks if I add another constructor that takes a Throwable cause?

Philippe Maquet: No, IMO. I interpret the instructions as this : the zero argument constructor and the one that takes a String must be there as a minimum.

How about if I made the zero argument and String constructors private?

[jiju ka]

Has any one else noticed that the order of the fields description within the database is not the name as the order of the records

My schema definition in file matches exactly with data layout. To be more specific my file have number of fields in each record, schema description section and thenn data section.
In schema description, there are length of field name, field name and field length for each field. Nothing more. The ordering is implicit.

In your case the ordering should be explicit. If so is there a field(s) specifying order?. Please put more light. Just curious.
[ November 03, 2004: Message edited by: jiju ka ]