public interface Externalizable extends Serializable 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 supercede 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.
I should have not posted this question on this forum, because sometimes it causes confusion. Like you some others might have gone through index and contents of the book as well exam objective list. Actually Externalization is not on the Exam. But what to say of simulators. It is hard to be 100% specific.
Anyways Serialization is on the exam list; Externalization is subclass of Serialization.