Implement Data Access Layer

[Richard Jackson]

Hi,all
I just consider to start data access layer implementation.
After looking many of posts from every professionals,I am not still able to sure a final schema.
Firstly,I do data access funtions in the Data class,and its assistant class is DataSchema class.
1)I create open()/getNumOfFields()/getFieldNameLen()/getFieldName() method,etc.
(*arrays are byte[] fieldNameLen,String[] fieldName,byte[] fieldLen)

public void open(String name) throws Exception{
try{
// open data file
raf=new RandomAccessFile(name,"rw");
// validate magic cookie
if(raf.readInt()!=MAGIC_COOKIE)
throw new Exception("Unvalid data file!");
}catch(Exception e){
e.printStackTrace();
}
}
public short getNumOfFields(){
try{
raf.skipBytes(4);
numFields=raf.readShort();
}catch(Exception e){e.printStackTrace();}
return numFields;
}
public byte[] getFieldNameLength(){
return fieldNameLen;
}
public String[] getFieldName(){
return fieldName;
}
public byte[] getFieldLength(){
        return fieldLen;
}  

2)I do the read/write operations in the DataSchema's constructor.

public DataSchema(){
     try{
byte[] fieldNameLen=new byte[numFields];
        String[] fieldName=new String[numFields];
        byte[] fieldLen=new byte[numFields];
// read schema info
for(int i=1;i<numFields;i++){
        fieldNameLen[I]=raf.readByte();
byte[] name=new byte[fieldNameLen];
raf.readFully(name);
fieldName[i]=new String(name,"US-ASCII");
fieldLen[i]=raf.readByte();
}
     }catch(Exception e){e.printStackTrace();}
}  

3)And then,I can invoke the public methods to get schema fields wherever I am.
Secondly,I have to read and write records in the Data class.
All that above are correct?
Do like this is suitable to Sun' request?
Please comment and discuss about it.
Regards,
Richard

[Bill Robertson]

IMO, if your data class is also going to implement all of the
main functions (i.e. book, add, delete, find/search,lock..etc) you may
want to move all file processing into a single class. Then place
a handle to that class in the Data class so the Data class can
retrieve relevant file information. Why seperate them? Well in essence
a class should have on main purpose. If you place both the
file handling and data operations in to the Data class you asking
a lot out of your Data class. I know some who have done this
and passed the SCJD, whether or not they were penalized for it we'll never
know.

[Richard Jackson]

Hi,Bill
Thanks for your advice.
I have already do some sample programs to put similar operations into same class.
For instance,I attempted to design my data access function.

> DBAccess interface - required interface in the specification
> Data class - implements DBAccess interface and provide main data access function.
> DataSchema class- read schema info from the .db file with getFieldLen(),getFieldName() methods,etc.
> DataIO class - read/write fixed length string as static methods of DataIO.
> DiscountRoom - provide read/write info for fields of discount room database with getName(),getLocation(),etc  methods.
  

In my design above,they looks like correct with integrity?
Regards,Richard

[Bill Robertson]

Looks like your on the right track

[Richard Jackson]

Thank you,Bill
Considering the Data class implementation (extends from DBAccess interface),there are some doubtful points,

// Modifies the fields of a record. The new value for field n
// appears in data[n]. Throws SecurityException
// if the record is locked with a cookie other than lockCookie.
public void updateRecord(long recNo, String[] data, long lockCookie)
throws RecordNotFoundException, SecurityException;
// Deletes a record, making the record number and associated disk
// storage available for reuse.
// Throws SecurityException if the record is locked with a cookie
// other than lockCookie.
public void deleteRecord(long recNo, long lockCookie)
throws RecordNotFoundException, SecurityException;
// Creates a new record in the database (possibly reusing a
// deleted entry). Inserts the given data, and returns the record
// number of the new record.
public long createRecord(String [] data)
throws DuplicateKeyException;

In the DBAccess interface,I have the following estimations,
1) The createRecord() and deleteRecord() method is used to changing the database ,so they are not relational with booking a room?
They don't have to exist in a remote interface ( such as 'interface
RemoteDBAccess').
2) In the update() method,what should I do to modify the value of field n to update the record?
3) "Deletes a record, making the record number and associated disk
storage available for reuse. "
According deleteRecord() method, how can I delete a record and make the disk space avaliable for reuse?
4) During the course of create/delete/update/find record data,is that necessary to wrap the records into a Collection (like List or Set) for easy operation?
Regards, Richard

[Jay Bromley]

I'm not working on the same exam as you, but the data access layer sounds very much the same. My thoughts on your questions:
1) In my project the people who book things are not the same people who would be adding and deleting things which can be booked, so I think the create and delete functionality stays buried in the data access layer. This functionality does not belong in the business logic for the customer service reps. I believe the evaluation involves some sort of automated testing of your data access layer, so the low level functionality of course must be complete.
2) Perhaps I'm not understanding, but you would probably just want to write the data back out to the file. If the nuts and bolts of this are the question, then the Sun tutorials provide a good speel on File I/O. A questin I'm trying to resolve is traditional I/O or NIO. I'm leaning towards traditional, but mostly because I don't yet know enough about NIO. After some investigation I'll have some better ideas.
3) I interpreted this to mean that the space of a record marked deleted can be used in the creation of another record. Considering point 1) this means your client will never reuse this space (because the people who book can't create records.) I think the wording is a little misleading, the idea is to allow the space to be reused by the app. I didn't read this to mean that you must make the space available to the OS.
Well, good luck and regards,
Jay Bromley

[Richard Jackson]

Hi,Jay
I understand what you said,and thank you help me.
But I still need do some practices.By the way,I am not able to get answer about No.4 question.

Richard:
4) During the course of create/delete/update/find record data,is that necessary to wrap the records into a Collection (like List or Set) for easy operation?

Who else can explain that for me?
Regards,
Richard

[Bill Robertson]

Richard:
4) During the course of create/delete/update/find record data,is that necessary to wrap the records into a Collection (like List or Set) for easy operation?

Where are you going with this? Whats the purpose? I am not saying
its wrong I am just not sure what the purpose is? Locking/Unlocking?

[Richard Jackson]

Hi,Bill

Bill:
I am not saying its wrong I am just not sure what the purpose is? Locking/Unlocking?
Richard:
4) During the course of create/delete/update/find record data,is that necessary to wrap the records into a Collection (like List or Set) for easy operation?

My meaning was whether I need to cache all records into a collection(like List).
I just saw many of people in the forum have done this step.
Regards,
Ricard

[Richard Jackson]

Hi,all
I have two problems about modify database.
1)In my above post in this topic,there is a DiscountRoom class provide some getters/setters methods for reading/writting field data.
When I have to create/delete/update records,should I add some other methods (like int getFieldNumber(),etc) to return field n?
2)Now that all fields read as a string,what should I do make a deleted record return its record number?
Regards,
Richard