Implementing Serializable

[Edward Lawrence]

The following code is my "java practice" versions of the Dog and Collar classes taken from the book "SCJP Sun Certified Programmer for Java 6 Study Guide (Exam 310-065)" Chapter 6 Certification Objectives—Serialization (Exam Objective 3.3).

I expected the code to produce a runtime exception because the Collar class did not implement Serializable, but the code compiles and runs producing the following output.

Collar@578ceb
5
yes

Question 1. Why didn't I get an exception?
Question 2. What exactly does the output Collar@578ceb represent?

import java.io.*;

class Dog implements Serializable {
   private Collar theCollar;
   private int dogSize;
   public Dog(Collar collar, int size) {
     theCollar = collar;
     dogSize = size;
   }
   public Collar getCollar() { return theCollar; }
}

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

public class DeSerializeDog2 {

	public static void main(String [] args) {
		//SerializeDog1 sd1 = new SerializeDog1();
		Collar collar = new Collar(5);
		Dog dog = new Dog(collar, 6);

		try{
			FileOutputStream fos = new FileOutputStream("zDog.ser");
			ObjectOutputStream oos = new ObjectOutputStream(fos);
			oos.writeObject(dog);
		} catch (IOException ioe) {

		}

		try {
			FileInputStream fis = new FileInputStream("zDog.ser");
			ObjectInputStream ois = new ObjectInputStream(fis);
			dog = (Dog)ois.readObject();

		} catch (IOException ioe) {

		} catch (ClassNotFoundException cnf) {

		}
		System.out.println(   dog.getCollar() );
		System.out.println(   dog.getCollar().getCollarSize()   );
		System.out.println("yes");

	}

}

[Jeff Verdegan]

Edward Lawrence wrote:
Question 1. Why didn't I get an exception?

Why do you think you didn't? How would you know if you did?

Question 2. What exactly does the output Collar@578ceb represent?

When you print out an object, the println() method ends up calling that object's toString() method. If you don't override toString(), you get Object's version. That's what you're seeing.

[Edward Lawrence]

I think I didn't get an exception because I got the output 5. However, I realized that I did not handle any possible caught exception in my catch block. I probably wrongly deduced that if 5 printed I did not get an exception. I will try it with printing any possible caught exception.

If I get an exception printed my question would be why did it produce the expected result?

[Edward Lawrence]

Yes. I did get the following exception with the expected results.

Question: Why did it go ahead and produce the correct results after the exception?

java.io.NotSerializableException: Collar
java.io.WriteAbortedException: writing aborted; java.io.NotSerializableException
: Collar
Collar@578ceb
5
yes

[Paul Clapham]

Edward Lawrence wrote:Question: Why did it go ahead and produce the correct results after the exception?

Because that's how your code is written. You catch the exception and handle it (by writing it to System.out, I suppose) in lines 39-43 and then continue to the next statements in lines 44-46, which do what you call "producing the correct results".

[Edward Lawrence]

Thank you Paul. I am relatively new to Java and I am leaning by writing exprimental code. I am closing this question.

[Jeff Verdegan]

Edward Lawrence wrote:Yes. I did get the following exception with the expected results.

Question: Why did it go ahead and produce the correct results after the exception?

It didn't.

The collar variable never got set to its new value because of the exception. Then you carried on after the exception as if everything was fine (even though it wasn't--the serialization/deserialization failed). Then you printed out some collar info, which still had its original value. It just so happens that collar having the original value looks exactly the same as collar after the results of a successful deserialization.

In other words, the particular incorrect results you had are indistinguishable from the desired results.

When an exception occurs, you need to either actually handle it (meaning you retry or use some default value or do something that can properly serve as "success" in that context), or else you don't catch it in the first place, or if you do, you log, wrap, and rethrow.

Merely catching an exception doesn't fix the problem.