Writing Java Objects to an XML file

apoch632 · 05-10-2009 03:18PM #1

Hello I am having a bit of trouble writing an Object Car to an XML file

It keeps giving me the following two errors
java.lang.InstantiationException: StorageFiles.Car
Continuing ...
java.lang.Exception: XMLEncoder: discarding statement XMLEncoder.writeObject(Car);

The code I am using to write to the xml file is

import StorageFiles.*;
import addFiles.*;
import java.io.*;
import java.beans.XMLEncoder;
public class SaveCarObjects {

public void saveCarFiles(Car c)
{
try{

XMLEncoder encoder = new XMLEncoder(new BufferedOutputStream(new FileOutputStream("cars.xml")));
encoder.writeObject(c);
}
catch(Exception e)
{
e.printStackTrace();
}
}

}

the add car method is as follows

//the different values are taken in.....
Person driver = new Person(personName,personAddress,vehicleReg,tag);
Car newCar = new Car(vehicleReg,driver);
SaveCarObjects ss = new SaveCarObjects();
ss.saveCarFiles(newCar);

//Have a couple of System.out.print s to see that the values are being taken in correctly to the newCar object which they seem to be as it is outputting what I enter correctly suing the getMthods in the Car and Person class files

Can anyone tell me what I am doing wrong? I can post the code for the classes Car and Person if needs be . Both are implementing Serializable

Oh and will it be possible to read the various Objects that I am trying to store in this file later into an array of Car

Mad_Max · 05-10-2009 06:09PM

Post the Car class

apoch632 · 05-10-2009 06:18PM

Car

public class Car extends VehicleType implements Serializable {
    
public Car(String reg, Person per)
{
   regPlate = reg;
   person = per;
}


    
}

VehicleType

import java.util.*;
import java.io.*;
public abstract class VehicleType implements Serializable{

    protected String regPlate;
    Person person;
    int[] date; //For the method when it goes throw the toll bridge

    public String getRegPlate()
    {
	return regPlate;
    }
    public void setRegPlate(String regPlateTakenIn)
    {
	regPlate = regPlateTakenIn;
    }
    public Person getPerson()
    {
        return person;
    }
    public void setPerson(Person p)
    {
        person = p;
    }

Person

public class Person implements Serializable{

    
    String name;
    String address;
    String carRegPlate;

        //Different PayementTypes
    NonRegisteredPayement nonPayement=null;
    TagPayement tagPayement=null;
    VideoPayement vidPayement=null;


    ///Constructor For Non Registered Payement
    public Person(String nam,String add,String carPlate,NonRegisteredPayement non)
    {
        nonPayement = non;
        name = nam;
        address = add;
        carRegPlate = carPlate;

    }
    public Person(String nam,String add,String carPlate,TagPayement tag)
    {
        tagPayement = tag;
        name = nam;
        address = add;
        carRegPlate = carPlate;
        
    }
    public Person(String nam,String add,String carPlate,VideoPayement vid)
    {
        vidPayement = vid;
        name = nam;
        address = add;
        carRegPlate = carPlate;

    }

    //Get and Set Methods
    public String getName()
    {
        return name;
    }
    public void setName(String s)
    {
        name = s;
    }
    public String getAddress()
    {
        return address;
    }
    public void setAddress(String s)
    {
        address = s;
    }
    public String getPlate()
    {
        return carRegPlate;
    }
    public void setPlate(String s)
    {
        carRegPlate = s;
    }

    public NonRegisteredPayement getNon()
    {
        return nonPayement;
    }
    public void setNon(NonRegisteredPayement non)
    {
        nonPayement = non;
    }
    public TagPayement getTag()
    {
        return tagPayement;
    }
    public void setTag(TagPayement t)
    {
        tagPayement = t;
    }
    public VideoPayement getVid()
    {
        return vidPayement;
    }
    public void setVid(VideoPayement vid)
    {
        vidPayement = vid;
    }
}

I have the various payement objects all serialized as well

apoch632 · 05-10-2009 06:41PM

I've rewritten it to pass the object to a data file

public void saveCarFiles(Car c)
    {
        ObjectOutputStream obj = null;
        
        try{
            obj = new ObjectOutputStream(new FileOutputStream("cars.dat",true));
            obj.writeObject(c);
            
        }
        catch(IOException e)
        {  
            e.printStackTrace();
        }
    }

it will write one object to a file but not any more(I think!! the size of the .dat file is not increasing when I keep calling my addCar method)

I just want it to write multiple car objects to a file and then to be able to read them back in from that file. how would I go about do that

apoch632 · 06-10-2009 04:54PM

Okay I now have it writing multiple car objects to a .dat file. And it can append on more cars as the addcar method is called.

I was just wondering how can i read these multiple objects and show up the output.
It reads in the first one fine but after that it throws a file read error. Ideally I want it to iterate through till it comes to the last object in the file. how would I go about doing that.

package ActionMethods;

/**
 *
 * @author apoch632
 */
import java.io.*;
import StorageFiles.*;
public class ReadCarObjects {
    private ObjectInputStream input;
    private static int i;

    public void openFile()
    {
        try {
            input = new ObjectInputStream(new FileInputStream( "cars.dat"));
        }
        catch(IOException ioException)
        {
            System.err.println("Error Opening File :");
        }
    }
    public void readCars()
    {
        
        Car c1;
        Car c2;

        System.out.println("Printing Car Record");
        try{
            
            while(true)
            {
                c1 = (Car) input.readObject();
                c2 = (Car) input.readObject();
               

                
                System.out.println(c1.getPerson().getName());
                System.out.println(c1.getPerson().getName());
                System.out.println(c1.getPerson().getAddress());
                System.out.println(c1.getPerson().getAccount().getAccBalance());
                
                System.out.println(c2.getRegPlate());
                System.out.println(c2.getPerson().getName());
                System.out.println(c2.getPerson().getAddress());
                System.out.println(c2.getPerson().getAccount().getAccBalance());


            }

            
            

        }
        catch(EOFException endOfFileException)
        {
            return;
        }
        catch(ClassNotFoundException classNotFoundex)
        {
            System.err.println("Unable to create Object");
        }
        catch(IOException ioEx)
        {
            System.err.println("Error During File Read");
        }

    }
    }

AARRRGH · 06-10-2009 06:18PM

Is this a college project or a proper application you're trying to develop? If it's the latter, you can get object persistance using something like Versant's object database.

apoch632 · 06-10-2009 06:19PM

College project

Civilian_Target · 06-10-2009 06:58PM

Ah, yeah, was going to say...
Is there some way to do easy XML serialisation inside the provided SDK? Because I am not aware of one, for debugger logs I use reflection and for production I use XStream.

apoch632 · 06-10-2009 07:00PM

I should perhaps ask for a title change. I just want to read multiple objects from a file now.

I can read the first object stored in the file fine but cannot read the second or any subsequent ones

hussey · 07-10-2009 11:19AM

apoch632 wrote: »

Hello I am having a bit of trouble writing an Object Car to an XML file

It keeps giving me the following two errors
java.lang.InstantiationException: StorageFiles.Car
Continuing ...
java.lang.Exception: XMLEncoder: discarding statement XMLEncoder.writeObject(Car);

The code I am using to write to the xml file is

Any object using XMLEncoder/decoder must have a non-argument constructor
like new Car() etc
ideally it will conform to java.beans standard ....

some alternative can be apache digester http://commons.apache.org/digester/
kinda like a mapping of xPath to teh get/set methods etc

or else use the Car class in an array or List etc and use XMLEncoder etc

Writing Java Objects to an XML file

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