Object serialization and deserialization when class field changed

Question

I have a question about Object serialization and deserialization when class field changed.

If an object with type MyClass

MyClass {
    String str1;
    LinkedList mylist = new LinkedList();
    String str2;

}

has been serialized to file.

Then I changed the code which changes MyClass definition to

MyClass {
    String str1;
    LinkedList mylist = new LinkedList();
    Map myMap = new HashMap();

}

After that, I deserialize the object from file to a MyClass object using the changed code. Is it OK? Will there any exception thrown during deserialization? I want to reuse the old object. I.e. I want the de-serializing can be done. So I hope there is no exception thrown.

Thanks.

Answer 1

No, on the contrary, you'll run into great problems. In Joshua Bloch's Effective Java, Item 74, he notes to implement Serializable judiciously, as it decreases the flexibility to change a class's implementation once its been released.

He specifically notes that "If you do not make the effort to design a custom serializable form, but merely accept the default, the serialized form will be forever tied to the class's original internal representation."

Answer 2

This is why you should have a serialVersionUID. It would be different between the two versions, and your code would know it can't be done, and stop the de-serializing.