This Externalizable interface extends Serializable interface. The Externalizable has two member functions readExternal and writeExternal.
If a class that implements Externalizable must have a default no-argument constructor.
Serialization takes place at writeExternal and Deserializaion takes place at readExternal.
Scenario can be if you would like to encrypt some value to be stored/persisted or you would like to decrypt the value before restoration. You can use the Externalizable.
In POJO class you need write logix for the writeExternal and readExternal because that class implements Externalizable interface.