Serialization: Non serialized class issue

[Ajit Sawant]

Hello,
This is a modified version of K&B code (SCJP6 pg 466). Instead of putting serialization logic in the serializable class Dog, I have put that in a separate class.
In the code below
1)For "System.out.println("After Deserialization dogSize value :" + d.dogSize);"
The output is 0, expecting it to be 5
2)For "System.out.println("After Deserialization collarSize value :" + d.getCollar().getCollarSize());" it gives error "java.lang.NullPointerException at SerializeCat2.main(SerializeCat2.java:97)
3) Also, in the private method writeObject is there any way to avoid using a temp variable?
Look like the object d is not getting deserialized. Please let me know why it is not working.
Thanks in advance.
Ajit
---

//Example when a class is not serializable
import java.io.*;
class Dog implements Serializable{
  transient private Collar theCollar; // class Collar is NOT serializable
  int dogSize;
  private String test;
 
//Constructor
  public Dog (Collar collar, int size){
    theCollar =collar; 
  }
  void SetValue(String xyz){
System.out.println("default value for test: "+ test);
test = xyz;
        System.out.println("Set the value of instance variable to " + test);
  }
  String GetValue(){
    return test;
  }
//Initializer
  {
test = "xyz";
  }
// static intializer
   static{
test2 = "xcv";
   }
   static String  test2 = "abc";  // This the output as it runs sequentially after static initialzer
// Methods logic
  public Collar getCollar(){
return theCollar;
  }
  
}
class Collar{
  private int collarSize;
//Constructor
  public Collar (int size){ 
collarSize =size;
  }
  public int getCollarSize(){ 
return collarSize;
  }
}
 
public class SerializeCat2{
   static int temp;
   private void writeObject(ObjectOutputStream os){
   try{
     
     os.defaultWriteObject();
     os.writeInt(temp); //Can using temp be avoided?
     //os.writeInt(d.getCollar().getCollarSize());
     //os.writeInt(c.getCollarSize());
   }
   catch (Exception e){e.printStackTrace();}
   }
   private void readObject(ObjectInputStream is){
   try{
     
     is.defaultReadObject();
     Collar c = new Collar (is.readInt());// just c is nor good need to  have Collar c
   }
   catch (Exception e){e.printStackTrace();}
   }
 
public static void main (String [ ] args)
       {
 
 
                Collar c = new Collar(3);
Dog d = new Dog(c,5);
                d.SetValue("bbb");
                System.out.println("New Value before Serialization:" + d.GetValue());
                System.out.println("Before  Serialization static Value :" + d.test2);
try{
FileOutputStream fs = new FileOutputStream("testSer2.ser");
ObjectOutputStream os =  new ObjectOutputStream(fs);
                        temp =d.getCollar().getCollarSize();
                        System.out.println("Value of Collarsize-temp :" +temp);
os.writeObject(d);
os.close();

}
catch(Exception e){
e.printStackTrace();
}
try{
FileInputStream fis = new FileInputStream("testSer2.ser");
ObjectInputStream ois =  new ObjectInputStream(fis);
d = (Dog)ois.readObject(); // Method returns Object so we need to cast deserialized object to Dog 
ois.close();
        System.out.println("After Deserialization Value :" + d.GetValue());
                        System.out.println("After Deserialization static Value :" + d.test2);
                        System.out.println("After Deserialization  dogSize value :" + d.dogSize);
                System.out.println("After Deserialization collarSize value :" + d.getCollar().getCollarSize());
}
catch(Exception e){
e.printStackTrace();
}
 
}
 
}

---

[ December 28, 2008: Message edited by: Ajit Sawant ]

[ December 28, 2008: Message edited by: Ajit Sawant ]
[ December 29, 2008: Message edited by: Ajit Sawant ]

[James Tharakan]

1) For "System.out.println("After Deserialization collarSize value :" + d.getCollar().getCollarSize());"
The output is 0, expecting it to be 5
2) For "System.out.println("After Deserialization dogSize value :" + d.dogSize);" it gives error "java.lang.NullPointerException at SerializeCat2.main(SerializeCat2.java:97)

Assuming that you wrote the answers of these question in wrong order


About the question 1
the output would have been 5, if you used the value that you passed to the method.What did you do with the size variable???

public Dog (Collar collar, int size){
theCollar =collar;
}

About question 2,
When deserialized, the value of transient variable of a serializble class is always set to its default value.
transient private Collar theCollar;

So, when the d.getCollar() of d.getCollar().getCollarSize(); returns,
it would be returning a null value.
Hope now you understand the execption.

About question 3,

The idea of using the temp is to store the value of a non-serializble variable collarSize;
Avoid the use of temp is not a problem , but that would create an unrequried method calling....


Hope i am clear and correct.

[Sunil Bangia]

if you require sun java voucher mail us............... [email protected]

[Ajit Sawant]

Hi James,
I have corrected the question's order and message mismatch.
Your repsonse

the output would have been 5, if you used the value that you passed to the method.What did you do with the size variable???
public Dog (Collar collar, int size){
theCollar =collar;
}

Where to use the value to pass it to a method? I didn't understand you question/hint. I expect os.defaultWriteObject() to care of Dog object as it is Serializable.

Your reponse

When deserialized, the value of transient variable of a serializble class is always set to its default value.
transient private Collar theCollar;
So, when the d.getCollar() of d.getCollar().getCollarSize(); returns,
it would be returning a null value.

As the Collar class is not serializable I'm witing it using os.writeInt(temp); //temp (has value 3) store the value for CollarSize. When I use Collar c = new Collar (is.readInt()). I expect it to read 3 create a new Collar object by using Collar c = new Collar (is.readInt()) in my code.
Then build the the Dog object again Dogsize 5 and collarSize 3 (new Collar created durinf deserializtion.
Please correct me if my understanding is wrong.
Thanks
Ajit

[James Tharakan]

When your Are creating the Dog object here, Dog d = new Dog(c,5);
You are sending two arguments,
And in the Dog constructor....

public Dog (Collar collar, int size){
theCollar =collar;
}

you took the argument into the parameter size.And you did not assign it to any variable(its wasted). It should be ussed like this.
public Dog (Collar collar, int size){
theCollar =collar;
dogSize=size;
}

Then build the the Dog object again Dogsize 5 and collarSize 3 (new Collar created durinf deserializtion.

private void readObject(ObjectInputStream is){
   try{
     
     is.defaultReadObject();
       Collar c = new Collar (is.readInt());// just c is nor good need to  have Collar c
    
   }
   catch (Exception e){e.printStackTrace();}
    
   }

In this method you created a new collar object.Which is actually meant for the Dog object's instance variable thecollar.
But nowhere in the code you have assigned this newly reconstructed collar object to the dog's instance variable thecollar

Hope you understood ..

[Ajit Sawant]

Now i did dogSize= size. But I'm able to deserialize Dog.
Unlke the example in the book, I can not do d.theCollar = new Collar(is.readInt()). How do set theCollar as it not in the same class file.
I created a new Collar object, created a new Dog object so that the variable "theCollar" should get set. But it is not working.
Q) Why should I create a new Dog object? Deserialization shpuld do it for me. By creating a new Dog object I'm calling the Dog constructor which should nt be executed as per deserialation compept. Any it did not work?
How can get the same Dog object I have the new Collar object now? Iwas expecting "theCollar" to set bet deserialization using the new Collar c.
I did the following

is.defaultReadObject();
     Collar c = new Collar (is.readInt());// just c is nor good need to  have Collar c
     Dog d = new Dog(c,tempdogSize);

Then tried to print the values
System.out.println("After Deserialization dogSize value :" + d.dogSize);
System.out.println("After Deserialization new Collar's collarSize: " + c.getCollarSize());
System.out.println("After Deserialization collarSize value :" + d.getCollar().getCollarSize());

The output is
After Deserialization dogSize value : 5
After Deserialization new Collar's collarSize: 3
java.lang.NullPointerException at SerializeCat2.main(SerializeCat2.java:105)
[ December 30, 2008: Message edited by: Ajit Sawant ]

[Ajit Sawant]

I made few change to make it run without error. I'm not that is the right way as I doing what I expect deserialization to do for me. I think it is not true deserialization
1) created a new meth od in Dog
public void setCollar(Collar collar)
{
theCollar =collar;
}
2)No explicit writing of Collar (os.writeInt(temp))

private void writeObject(ObjectOutputStream os){
   try{
         os.defaultWriteObject();
..

3) No new Collar object creation ( Collar c = new Collar (is.readInt()) )

   private void readObject(ObjectInputStream is){
   try{
     
     is.defaultReadObject();

4)After deserialization statement d = (Dog)ois.readObject();
d.setCollar(c);

This works. It will also work if I do Collar c = new Collar (is.readInt())
in readObject(ObjectInputStream i).

In both cases is it a true deserialization? as I'm setting the value for "theCollar" after executing the deserialization d = (Dog)ois.readObject();