You can follow this link
http://www.smartdataprocessing.com/Serialization.htm The advantage of externalizable is that you have the control over which fields to serialize/de-serialize.
By default, serialization serializes all the fields that are non-transient and non-static and implements Serializable. So, if you want to override this serialization process governing exactly what fields to serialize then you can implement Externalizable.
However, you can get a plethora of information googling serializable and externalizable