ObjectInput Stream & objectOutputStr eam

[khajeddin]

hi:
i have a program which should read and werite on a file but after the first time it writeon the file it can read te records.but other times it write on the file but just read the records which has written first time. here is my programs:

 
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.NoSuchElementException;
import java.util.Scanner;
 
import com.deitel.jhtp7.ch14.AccountRecordSerializable;
 
public class CreateSequentialFile
{
   private ObjectOutputStream output; // outputs data to file
 
   // allow user to specify file name
   public void openFile()
   {
      try // open file
      {
         output = new ObjectOutputStream(
            new FileOutputStream( "clients.ser" ,true) );
      } // end try
      catch ( IOException ioException )
      {
         System.err.println( "Error opening file." );
      } // end catch
   } // end method openFile
 
   // add records to file
   public void addRecords()
   {
      AccountRecordSerializable record; // object to be written to file
      int accountNumber = 0; // account number for record object
      String firstName; // first name for record object
      String lastName; // last name for record object
      double balance; // balance for record object
 
      Scanner input = new Scanner( System.in );
 
      System.out.printf( "%s\n%s\n%s\n%s\n\n",
         "To terminate input, type the end-of-file indicator ",
         "when you are prompted to enter input.",
         "On UNIX/Linux/Mac OS X type <ctrl> d then press Enter",
         "On Windows type <ctrl> z then press Enter" );
 
      System.out.printf( "%s\n%s", 
         "Enter account number (> 0), first name, last name and balance.",
         "? " );
 
      while ( input.hasNext() ) // loop until end-of-file indicator
      {
         try // output values to file
         {
            accountNumber = input.nextInt(); // read account number
            firstName = input.next(); // read first name
            lastName = input.next(); // read last name
            balance = input.nextDouble(); // read balance
 
            if ( accountNumber > 0 )
            {
               // create new record
               record = new AccountRecordSerializable( accountNumber,
                  firstName, lastName, balance );
               output.writeObject( record ); // output record
            } // end if
            else
            {
               System.out.println(
                  "Account number must be greater than 0." );
            } // end else
         } // end try
         catch ( IOException ioException )
         {
            System.err.println( "Error writing to file." );
            return;
         } // end catch
         catch ( NoSuchElementException elementException )
         {
            System.err.println( "Invalid input. Please try again." );
            input.nextLine(); // discard input so user can try again
         } // end catch
 
         System.out.printf( "%s %s\n%s", "Enter account number (>0),",
            "first name, last name and balance.", "? " );
      } // end while
   } // end method addRecords
 
   // close file and terminate application 
   public void closeFile() 
   {
      try // close file
      {
         if ( output != null )
            output.close();
      } // end try
      catch ( IOException ioException )
      {
         System.err.println( "Error closing file." );
         System.exit( 1 );
      } // end catch
   } // end method closeFile
} // end class CreateSequentialFile
 
 

 
import java.io.EOFException;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
 
import com.deitel.jhtp7.ch14.AccountRecordSerializable;
 
public class ReadSequentialFile
{
   private ObjectInputStream input;
 
   // enable user to select file to open
   public void openFile()
   {
      try // open file
      {
         input = new ObjectInputStream(
            new FileInputStream( "clients.ser" ) );
      } // end try
      catch ( IOException ioException )
      {
         System.err.println( "Error opening file." );
      } // end catch
   } // end method openFile
 
   // read record from file
   public void readRecords()
   {
      AccountRecordSerializable record;
      System.out.printf( "%-10s%-12s%-12s%10s\n", "Account",
         "First Name", "Last Name", "Balance" );
 
      try // input the values from the file
      { 
         while ( true )
         {
            record = ( AccountRecordSerializable ) input.readObject();
 
            // display record contents
            System.out.printf( "%-10d%-12s%-12s%10.2f\n",
               record.getAccount(), record.getFirstName(),
               record.getLastName(), record.getBalance() );
         } // end while
      } // end try
      catch ( EOFException endOfFileException )
      {
         return; // end of file was reached
      } // end catch
      catch ( ClassNotFoundException classNotFoundException )
      {
         System.err.println( "Unable to create object." );
      } // end catch
      catch ( IOException ioException )
      {
         System.err.println( "Error during reading from file." );
      } // end catch
   } // end method readRecords
 
   // close file and terminate application
   public void closeFile()
   {
      try // close file and exit
      {
         if ( input != null )
            input.close();
         System.exit( 0 );
      } // end try
      catch ( IOException ioException )
      {
         System.err.println( "Error closing file." );
         System.exit( 1 );
      } // end catch
   } // end method closeFile
} // end class ReadSequentialFile
 
 

what should i do to always read all records ?

[r035198x]

hi:
i have a program which should read and werite on a file but after the first time it writeon the file it can read te records.but other times it write on the file but just read the records which has written first time. here is my programs:

 
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.NoSuchElementException;
import java.util.Scanner;
 
import com.deitel.jhtp7.ch14.AccountRecordSerializable;
 
public class CreateSequentialFile
{
private ObjectOutputStream output; // outputs data to file
 
// allow user to specify file name
public void openFile()
{
try // open file
{
output = new ObjectOutputStream(
new FileOutputStream( "clients.ser" ,true) );
} // end try
catch ( IOException ioException )
{
System.err.println( "Error opening file." );
} // end catch
} // end method openFile
 
// add records to file
public void addRecords()
{
AccountRecordSerializable record; // object to be written to file
int accountNumber = 0; // account number for record object
String firstName; // first name for record object
String lastName; // last name for record object
double balance; // balance for record object
 
Scanner input = new Scanner( System.in );
 
System.out.printf( "%s\n%s\n%s\n%s\n\n",
"To terminate input, type the end-of-file indicator ",
"when you are prompted to enter input.",
"On UNIX/Linux/Mac OS X type <ctrl> d then press Enter",
"On Windows type <ctrl> z then press Enter" );
 
System.out.printf( "%s\n%s", 
"Enter account number (> 0), first name, last name and balance.",
"? " );
 
while ( input.hasNext() ) // loop until end-of-file indicator
{
try // output values to file
{
accountNumber = input.nextInt(); // read account number
firstName = input.next(); // read first name
lastName = input.next(); // read last name
balance = input.nextDouble(); // read balance
 
if ( accountNumber > 0 )
{
// create new record
record = new AccountRecordSerializable( accountNumber,
firstName, lastName, balance );
output.writeObject( record ); // output record
} // end if
else
{
System.out.println(
"Account number must be greater than 0." );
} // end else
} // end try
catch ( IOException ioException )
{
System.err.println( "Error writing to file." );
return;
} // end catch
catch ( NoSuchElementException elementException )
{
System.err.println( "Invalid input. Please try again." );
input.nextLine(); // discard input so user can try again
} // end catch
 
System.out.printf( "%s %s\n%s", "Enter account number (>0),",
"first name, last name and balance.", "? " );
} // end while
} // end method addRecords
 
// close file and terminate application 
public void closeFile() 
{
try // close file
{
if ( output != null )
output.close();
} // end try
catch ( IOException ioException )
{
System.err.println( "Error closing file." );
System.exit( 1 );
} // end catch
} // end method closeFile
} // end class CreateSequentialFile
 
 

 
import java.io.EOFException;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
 
import com.deitel.jhtp7.ch14.AccountRecordSerializable;
 
public class ReadSequentialFile
{
private ObjectInputStream input;
 
// enable user to select file to open
public void openFile()
{
try // open file
{
input = new ObjectInputStream(
new FileInputStream( "clients.ser" ) );
} // end try
catch ( IOException ioException )
{
System.err.println( "Error opening file." );
} // end catch
} // end method openFile
 
// read record from file
public void readRecords()
{
AccountRecordSerializable record;
System.out.printf( "%-10s%-12s%-12s%10s\n", "Account",
"First Name", "Last Name", "Balance" );
 
try // input the values from the file
{ 
while ( true )
{
record = ( AccountRecordSerializable ) input.readObject();
 
// display record contents
System.out.printf( "%-10d%-12s%-12s%10.2f\n",
record.getAccount(), record.getFirstName(),
record.getLastName(), record.getBalance() );
} // end while
} // end try
catch ( EOFException endOfFileException )
{
return; // end of file was reached
} // end catch
catch ( ClassNotFoundException classNotFoundException )
{
System.err.println( "Unable to create object." );
} // end catch
catch ( IOException ioException )
{
System.err.println( "Error during reading from file." );
} // end catch
} // end method readRecords
 
// close file and terminate application
public void closeFile()
{
try // close file and exit
{
if ( input != null )
input.close();
System.exit( 0 );
} // end try
catch ( IOException ioException )
{
System.err.println( "Error closing file." );
System.exit( 1 );
} // end catch
} // end method closeFile
} // end class ReadSequentialFile
 
 

what should i do to always read all records ?

Don't you get an exception when you run this?

[r035198x]

Don't you get an exception when you run this?

Don't you get an exception when you run this?


B.T.W: I've now switched to FF.

[dmjpro]

Do close all the streams before reuse it.
I think it ll help u.
Best of luck.

Kind regards,
Dmjpro.

[JosAH]

hi:
i have a program which should read and werite on a file but after the first time it writeon the file it can read te records.but other times it write on the file but just read the records which has written first time.

Your problem is this: everytime you open an ObjectOutputStr eam the stream
writes header information. When that is ready you can write objects to the
stream.

You do this: you open a file, write one object and close the file again. The file
now contains this:

HHHOOOOOOO ... OOO

where the 'H's represent the header and the 'O' represents the serialized object.

Then you do it again; the contents of the file now is:

HHHOOOOOOO ... OOOHHHOOOOOOO ... OOO

Where you expect your second object, a second header is written and when
you attempt to read an object you'll get a exception telling you that the stream
is corrupted.

There are two solutions to this problem:

1) you either keep your file and ObjectOutputStr eam open all the time so only
when the ObjectOutputStr eam is created once that header will be written.

or

2) You implement this class:

public class YourObjectOutputStream extends ObjectOutputStream {
 
   private boolean header;
 
   public YourObjectOutputStream(OutputStream out, boolean header) { 
      super(out); 
      this.header= header;
   }
 
   protected void writeStreamHeader() {
 
      if (header) super.writeStreamHeader(); 
   }
}
 

Only the first time you create your file you instantiate this class with the header
flag set to true; all the other instantiations (when you reopen your file again) the
header flag should be set to false. Read the API documentation for the description
of the writeStreamHead er() method.

kind regards,

Jos

[r035198x]

Your problem is this: everytime you open an ObjectOutputStr eam the stream
writes header information. When that is ready you can write objects to the
stream.

You do this: you open a file, write one object and close the file again. The file
now contains this:

HHHOOOOOOO ... OOO

where the 'H's represent the header and the 'O' represents the serialized object.

Then you do it again; the contents of the file now is:

HHHOOOOOOO ... OOOHHHOOOOOOO ... OOO

Where you expect your second object, a second header is written and when
you attempt to read an object you'll get a exception telling you that the stream
is corrupted.

There are two solutions to this problem:

1) you either keep your file and ObjectOutputStr eam open all the time so only
when the ObjectOutputStr eam is created once that header will be written.

or

2) You implement this class:

public class YourObjectOutputStream extends ObjectOutputStream {
 
   private boolean header;
 
   public YourObjectOutputStream(OutputStream out, boolean header) { 
      super(out); 
      this.header= header;
   }
 
   protected void writeStreamHeader() {
 
      if (header) super.writeStreamHeader(); 
   }
}
 

Only the first time you create your file you instantiate this class with the header
flag set to true; all the other instantiations (when you reopen your file again) the
header flag should be set to false. Read the API documentation for the description
of the writeStreamHead er() method.

kind regards,

Jos

And if they'd checked the console they'd have seen the exception ...

[JosAH]

And if they'd checked the console they'd have seen the exception ...

Well, this behaviour can look a bit obscure if one didn't read the API documentation
for those ObjectStreams. People just happily hack away, expecting that all
classes should work as they more or less think they should work; until something
goes wrong: they simply panic then and don't read because they haven't read
ever before ;-)

kind regards,

Jos