Simple Serializable demo with problems

Question

Hello!
I am having a very simple demo where I tried to see how the Serializable interface works. However, I am receiving message NotSerializableException. What's wrong?
Thanks in advance!
Kind regards,
Igor
Here's the demo:
import java.io.*;
public class SerializationDemo {
  private class Data implements Serializable {
    String someString = "This is a string";
    double someDouble = 1;
  public SerializationDemo() {
    try {
      FileOutputStream fos = new FileOutputStream("dataObjectSerialized");
      ObjectOutputStream oos = new ObjectOutputStream(fos);
      oos.writeObject(new Data());
      fos.close();
    catch (Exception e) {
      System.out.println(e.toString());
  static public void main(String args[]) {
    SerializationDemo demo = new SerializationDemo(); 
};

Answer 1

Consider the following code. What do you expect Child.foo() to print? Why? Supposing you made it serializable, how would you expect it to behave when deserialized?
public class Illustration {
   public Illustration(int x) {
      this.x = x;
      this.c = new Child();
   public void foo() {
      c.foo();
   private Child c;
   private int x;
   private class Child {
      public Child() {
      public void foo() {
         System.out.println(x);
   public static void main(String[] argv) {
      Illustration i = new Illustration(42);
      i.foo();
}In case that's not clear, here's a quick illustration:
public class Illustration {
   public class Child { ... }
+--------------+
| Illustration |
|              |
|  +-----+     |
|  |Child|     |
|  +-----+     |
|              |
+--------------+
public class Illustration {
   public static class Child { ... }
+--------------+
| Illustration |    +-----+
|              |--->|Child|
|              |    +-----+
|              |
|              |
|              |
+--------------+Because Child is not declared static, it has full access to all of the fields of Illustration. Therefore if you serialize it in order to work after deserialization as it did before, it must have copies of all of the owning class's fields.
When you declare it static you break this relationship and it becomes independent.
So taking your following question:
Why is this so? Does this mean that the following two statements would produce completely identical results, providing that SerializationDemo class was made Serializable:
oos.writeObject(new Data());
oos.writeObject(this);No, not quite - the first form creates a new Data object, serializes that, then (of necessity) serializes the owning class as well.
The second form just serializes the current class - since it doesn't have an associated Data object, nothing else gets serialized.
To achieve the effect you want, either make your Data class static, or declare it in its own file (which is roughly the same thing).
Dave.