2.3 The writeObject Method

For serializable objects, the writeObject method allows a class to control the serialization of its own fields. Here is its signature:

    private void writeObject(ObjectOutputStream stream)
        throws IOException;

Each subclass of a serializable object may define its own writeObject method. If a class does not implement the method, the default serialization provided by defaultWriteObject will be used. When implemented, the class is only responsible for writing its own fields, not those of its supertypes or subtypes.

The class's writeObject method, if implemented, is responsible for saving the state of the class. Either ObjectOutputStream's defaultWriteObject or writeFields method must be called once (and only once) before writing any optional data that will be needed by the corresponding readObject method to restore the state of the object; even if no optional data is written, defaultWriteObject or writeFields must still be invoked once. If defaultWriteObject or writeFields is not invoked once prior to the writing of optional data (if any), then the behavior of instance deserialization is undefined in cases where the ObjectInputStream cannot resolve the class which defined the writeObject method in question.

The responsibility for the format, structure, and versioning of the optional data lies completely with the class.