Only the identity of the class of an Externalizable instance is written in the serialization stream and it is the responsibility of the class to save and restore the contents of its instances.

  Only the identity of the class of an Externalizable instance is
written in the serialization stream and it is the responsibility
of the class to save and restore the contents of its instances.

The writeExternal and readExternal methods of the Externalizable
interface are implemented by a class to give the class complete
control over the format and contents of the stream for an object
and its supertypes. These methods must explicitly
coordinate with the supertype to save its state. These methods supersede
customized implementations of writeObject and readObject methods.


Object Serialization uses the Serializable and Externalizable
interfaces. Object persistence mechanisms can use them as well. Each
object to be stored is tested for the Externalizable interface. If
the object supports Externalizable, the writeExternal method is called. If the
object does not support Externalizable and does implement
Serializable, the object is saved using
ObjectOutputStream.
When an Externalizable object is
reconstructed, an instance is created using the public no-arg
constructor, then the readExternal method called. Serializable
objects are restored by reading them from an ObjectInputStream.


An Externalizable instance can designate a substitution object via
the writeReplace and readResolve methods documented in the Serializable
interface.