java.io.Serializable interface is an empty interface with no methods
and attributes.It is implemented by objects which are needed to be
serialized and serves only to identify the semantics of being
serializable. When process of serializing an object is to be
controlled then Externalizable interface is used.The Externizable
interface extends Serializable and adds two methods,writeExternal() and
readExternal() which are automatically called during serialization and
deserialization. Classes that require special handling during the
serialization and deserialization process must implement special
methods with these exact signatures: private void writeObject(java.io.ObjectOutputStream out) throws IOException
