(B&S)My Data.java and read() !

[Saheed Adepoju]

Hi all
First of all, if i violate any of the JavaRanch policies by dropping my Data.java's constructor and also my read() method for your advice and tips, pls remove appropriately and i apologise; but i just wanna know if my read() is on track and appropriate! I use a cache:

package suncertify.db;
 
import java.io.*;
import suncertify.db.*;
import java.util.*;
 
/**
 * A Data object contains all of the contractors records in the
 * data file in the system and operations that acts upon
 * the contractor objects.
 * Consists also of methods which allow users to read, update,create,delete
 * and find contractor records from the data file.
 * 
 * @author Saheed Adepoju
 * @version 2.1.1
 */
 
public class Data implements DB {
    /**
     * A static structure that keeps all the contractor records
     * contained in the datafile
     */
    private static List<Contractor> contractorRecords = new ArrayList<Contractor>();
 
    /**
     * A value that validates the data file as the valid data file
     */
    final private static long MAGIC_COOKIE = 513;
    
    /**
     * A value indicating where the records in the
     * data file starts
     */
    private static final int OFFSET = 70;
 
    /**
     * A variable that holds the FieldNames
     * of the records contained in the data file
     */    
    private static String fieldNames[];
 
    /**
     * The RandomAccess File that points to the data file
     * that contains the Contractor records
     */
    private static RandomAccessFile raf;
 
    /**
     * A variable that holds the recordLength of the dataFile;
     *
     */
    private static int recordLength = 0;//holds the recordLength
 
    /**
     * A value that refers to the name of the datafile
     *
     */
    private String dbName = new String();//holds the name of the DataFile
 
    /**
     * A value that holds that holds the LockCookie generated   
     */
    private static long lockCookie = 0;
 
    /**
     * Default constructor
     */
 
    public Data() {
    }
 
    /**
     * Creates an Instance of the Data class
     * Populates the cache and initialises the state of records
     * @param dbName Holds the name of the Data File
     */
 
    public Data(String dbName) {
        this.dbName = dbName;
        openDataFile();//open the data file
        validateDataFile(); //check if data file is valid if 
//not throw sub-class of RuntimeException(DataAccessException)       
        recordLength = getRecordLength();
        fieldNames = getFieldNames();
        contractorRecords = getRecords();
    
    }
 
    /**
     * Opens Data File
     * @throws DataAccessException if there is an Error
     */
 
    private void openDataFile() {
        try {
            File file = new File(dbName);
            raf = new RandomAccessFile(file,"rw");        
        }catch(IOException io){
            throw new DataAccessException("I/O Error");
        }
    }
 
 
    /**
     * Generate a unique value for the lockCookie
     * @return A<code>Long</code> a value that
     * contains a lockCookie
     */
 
    private long getLockCookie() {
 
    return ++lockCookie;
    }
 
 
    /**
     * Checks whether data File is a valid one
     *      
     */
 
    private static void validateDataFile() {
        int magic = 0;
        try {
            magic = raf.readInt();//reads the magic value of Data file
            if(magic!=MAGIC_COOKIE) {
                throw new DataAccessException("Wrong Data File");
            }
 
        } catch(IOException e) {
            throw new DataAccessException("I/O Error");
 
        }  
        
    }
 
    private static int getRecordLength() {
        int recordLength = 0;
        try {
            recordLength = raf.readInt();
        } catch(IOException io) {
            throw new DataAccessException("I/O Error");
        }
 
    return recordLength;
    }
 
    private static String[] getFieldNames() {
        int fieldsize;
        String []names;
 
       //go thru the data file and get field names
 
                               
    return names;
    }
 
 
 
    private static List<Contractor> getRecords() {
        int recordCount = 0;
        byte flag = 0;    
        Contractor contractor = null;
        List<Contractor> records = new ArrayList<Contractor>();
    try {
        while(true) {
            
                 //read all records and create contractor objects
               
                    records.set(recordCount,contractor);
                    recordCount++;
                } else {
                    recordCount++;
        
                }
 
            }
     
     }catch(IOException io) {
    throw new DataAccessException("I/O Error");
     }    
 
    return records;
    }
 
    public String[] read(int recNo) throws RecordNotFoundException {
 
        String [] record = null;
        Contractor contractor = null;
        //check if the record exist or throw RecordNotFoundException
 
        if(recNo <=0) {
            throw new RecordNotFoundException("No Such Record");  
        }else if(contractorRecords.get((recNo - 1)) == null) {
            throw new RecordNotFoundException("Record doesnt exist"); 
        }
        else {
        //Controlling if another client holds the recNo. 
        //Ensures only one client has the lock
            synchronized(contractorRecords) {
                //Checks if recNo. still exists. If No 
                //throws RecordNotFoundException
                if(contractorRecords.get(recNo - 1) == null) {
                    throw new RecordNotFoundException("Record doesnt exist");
                }
                    
                    contractor = contractorRecords.get((recNo - 1));
                    String [] records = { contractor.getName(),contractor.getLocation(),
                                contractor.getSpecialties(),contractor.getSize(),
                                contractor.getRate(),contractor.getOwner() };
                
                    record = records;
            }
 
        }
    
 
    return record;
    }
 
    public long lock(int recNo) throws RecordNotFoundException {
        long lockCookie = getLockCookie();
 
        return lockCookie;
    }
}

I dont know if this procedure is sufficient for a read() method? Also i dont know if my Data.java constructor is appropriate? Thanks! :roll:

[Andrew: removed code that is irrelevant to the discussion]
[ December 30, 2005: Message edited by: Andrew Monkhouse ]

[Ali Hussain]

For exception handling, I would have done something like this:

try {
recordLength = raf.readInt();
} catch(IOException io) {
throw new DataAccessException("I/O Error", io);
}

Note io is given as an argument to DataAccessException. In this way, exception chaining (getCause) can be used later.

[Andrew Monkhouse]

Hi Saheed

First of all, if i violate any of the JavaRanch policies by dropping my Data.java's constructor and also my read() method for your advice and tips, pls remove appropriately and i apologise;

This is dangerously close to having all code removed, however I have decided to leave it for now.

I think you have far too much code posted for getting many comments - it just becomes too much for others to read and make comments on. You would be better breaking it down to smaller sections. While I got rid of many methods and variables that were not germaine to your desired discussion on the read() methods and the constructor, it could have been pared down even further. Probably it would have been best to have a post that only discussed the constructor (and only had those methods and variables necessary for that discussion), then another post that only discussed the read method with a comment that global variables are initialized in the constructor (possibly with a pointer back to the original thread).

I don't think there is much value in having both a Javadoc comment and an inline comment:

    /**
     * A value that refers to the name of the datafile
     *
     */
    private String dbName = new String();//holds the name of the DataFile

Speaking of dbName - why is it a global variable? Do you use it outside of the constructor and it's associated openDataFile() method?

Your read() method has a very small window of oportunity for a record to be deleted between the validation of the record and the conversion back into a String[].

Regards, Andrew

[Saheed Adepoju]

Hi
Thanks Ali Hussein and Andrew for your insights! I apologise and would make necessary adjustment to posts i send next time! I see your point about the dbName, it's best made a local variable! I also see your point as regards the read()! I will look thru it again! I have modified the code to have a method to check for the validity of the file now! Thanks for your input!