Java Programming

I have read the doc. on both interfaces but I don't know exactly the difference between
Serializable and Externalizable.
I know that using Serializable you don't have to take care of the serialization yourself
but in Externalizable you have to.
But if you implement the read/writeObject in Serializable then isn't that the same
thing as to implement Externalizable and read/writeExternal?
Also, how come that the first non-serializable super class of a serilizable class need
to have a empty constructor?
Regards
/Fredrik