Do you understand what Serializable is for? Externalizable is similar, but it gives you complete control over the format of the serialized data for a class -- i.e., you have to save all the data yourself.
Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods. You can find a sample in here.
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