Serialization query using writeObject() & readObject() method to add Transient variable

[Vishal Hegde]

Hi,


I have Serialization query using writeObject() & readObject() method to add Transient variable, but its giving null pointer exception for "after: collar size is please advice where I am going wrong.

import java.io.*;
class Dog implements Serializable
{
private transient Collar theCollar;
private int dogSize;
public Dog(Collar collar,int size){
  theCollar=collar;
  dogSize=size;
}
public Collar getCollar(){return theCollar;}
private void writeObject(ObjectOutputStream os)
{
try 
   {
       os.defaultWriteObject();
	   os.writeInt(theCollar.getCollarSize());
   }
 catch(Exception e)
   {
       e.printStackTrace();
   }   
}
public void readObject(ObjectInputStream is)
{
  try{
   is.defaultReadObject();
   theCollar=new Collar(is.readInt());
     }
	 catch(Exception e){
	   
	     e.printStackTrace();
	   }
}
}
class Collar implements Serializable{

private int collarSize;
public Collar(int size){collarSize=size;}
public int getCollarSize(){return collarSize;}
}

class createDog
{
  public static void main(String args[])
  {
    Collar c=new Collar(3);
	Dog d =new Dog(c,8);
	System.out.println("before: collar size is:"+d.getCollar().getCollarSize());
	try
	 {
	   FileOutputStream fs=new FileOutputStream("dogState.ser");
	   ObjectOutputStream os=new ObjectOutputStream(fs);
	   os.writeObject(d);
	   os.close();
	 }
	 catch(Exception e)
	 {
	 e.printStackTrace();
	 }
	 try
	 {
	   FileInputStream fis=new FileInputStream("dogState.ser");
	   ObjectInputStream is=new ObjectInputStream(fis);
	   d=(Dog)is.readObject();
	   System.out.println("after: collar size is:"+d.getCollar().getCollarSize());
	   fis.close();
	 }
	 catch(Exception e){e.printStackTrace();}
  }
 } 

[Haresh Kumar]

Vishal Hegde wrote:Hi,

I have Serialization query using writeObject() & readObject() method to add Transient variable, but its giving null pointer exception for "after: collar size is please advice where I am going wrong.

Transient variable cant be serialize ..ok and when you deserialize your dog object at that time collers value is null.so nullpointerexception is ....

[Winston Gutkowski]

Haresh Kumar wrote:Transient variable cant be serialize ..

Haresh,

1. Welcome to JavaRanch.
2. You don't need to quote OP's entire post when you reply; it makes threads incredibly long. Just include the relevant portion.
3. Your reply should go outside the 'quote' block.

I've corrected yours this time.

Winston

[Winston Gutkowski]

Vishal Hegde wrote:I have Serialization query using writeObject() & readObject() method to add Transient variable, but its giving null pointer exception for "after: collar size is please advice where I am going wrong.

Vishal, could you please show us the entire error message exactly as you get it; otherwise we're just guessing.

One thing I did notice: Dog.readObject() should be private.

Also: those try...catch blocks of yours may be causing problems, since they don't halt execution. With something like serialization, it's much better to just let the program fail. Indeed, until you're sure that everything's working, I wouldn't use any try...catch blocks.

Winston

[Vishal Hegde]

Winston Gutkowski wrote:

Vishal Hegde wrote:I have Serialization query using writeObject() & readObject() method to add Transient variable, but its giving null pointer exception for "after: collar size is please advice where I am going wrong.

Vishal, could you please show us the entire error message exactly as you get it; otherwise we're just guessing.

One thing I did notice: Dog.readObject() should be private.

Also: those try...catch blocks of yours may be causing problems, since they don't halt execution. With something like serialization, it's much better to just let the program fail. Indeed, until you're sure that everything's working, I wouldn't use any try...catch blocks.

Winston

I am getting the below exception but I am using writeObject() & readObject() so that it will even take the state of transient value at a time, still I am getting NULL Pointer Exception.

D:\Java practise>java createDog
before: collar size is:3
java.lang.NullPointerException
        at createDog.main(createDog.java:65)

[Jeff Verdegan]

Vishal Hegde wrote:
I am getting the below exception but I am using writeObject() & readObject() so that it will even take the state of transient value at a time, still I am getting NULL Pointer Exception.

D:\Java practise>java createDog
before: collar size is:3
java.lang.NullPointerException
        at createDog.main(createDog.java:65)

1. Make writeObject() and readObject() private as Winston suggested.

2. Add a println() to make sure that d is not null, and that it's getCollar() that's returning null. I know it seems obvious that this must be the case, but we must always check our assumptions to eliminate paths of investigation.

3. Add println() calls to writeObject() and readObject() so you can see what they're doing.

[William Brogden]

Maybe I missed it.

Exactly where in that code are you providing a special initialization of the Collar reference "theCollar"?

As a transient reference you expect that the value should be null just like a transient int will be 0 on deserialization.

There are plenty of tutorials - here is a recent one.

Bill

[Mike Simmons]

Bill, theCollar is being set in the readObject method.

For everyone: Winston's first comment nails the issue. The readObject method needs to be private, as specified in the API for java.util.Serializable When you make it public, it doesn't get called. No, it makes no sense by any modern understanding of how Java should work. But that's how serialization was implemented in Java, back in the dawn of time.

[Vishal Hegde]

Thanks I changed the the method to private and now getting the result thnks