The I/O Package - The Java Programming Language - page 823

private void readObjectNoData() throws ObjectStreamException
If, as an object is deserialized, the serialized data lists the superclass as
a known superclass then the superclass's readObject method will be in-
voked (if it exists), otherwise the superclass's readObjectNoData method
will be invoked. The readObjectNoData method can then set appropriate
values in the object's superclass fields.
20.8.6. Serialized Fields
The default serialization usually works well, but for more sophisticated
classes and class evolution you may need to access the original fields.
For example, suppose you were representing a rectangle in a geometric
system by using two opposite corners. You would have four fields: x1 , y1 ,
x2 , and y2 . If you later want to use a corner, plus width and height, you
would have four different fields: x , y , width , and height . Assuming default
serialization of the four original fields you would also have a compat-
ibility problem: the rectangles that were already serialized would have
the old fields instead of the new ones. To solve this problem you could
maintain the serialized format of the original class and convert between
the old and new fields as you encounter them in readObject or writeOb-
ject . You do this using serialized field types to view the serialized form
as an abstraction and to access individual fields:
public class Rectangle implements Serializable {
private static final
long serialVersionUID = -1307795172754062330L;
private static final
ObjectStreamField[] serialPersistentFields = {
new ObjectStreamField("x1", Double.TYPE),
new ObjectStreamField("y1", Double.TYPE),
new ObjectStreamField("x2", Double.TYPE),
new ObjectStreamField("y2", Double.TYPE),
};
private transient double x, y, width, height;