Help with serialization

as part of a project at uni i have been asked to provide object persistance
through files

I have opted for serialization

my question is quite simple

when u use serialization is an object saved to a single file or are more
than one object(s) saved to a single file?

say i have three objects, will they all be saved into one file or will there
be three seperate files each having one object saved into them?

also say i have a superclass implmenting the java.io.Serializable

will all subclasses of that superclass automatically implement the
java.io.Serializable or will i need to make those subclasses implement the
Serializable seperately?

any help would be appreciated

PS -
public class UserData implements java.io.Serializablei understand that
but..public class UserData extends User...how would i implement the
java.io.Serializable in that class? would it be...public class UserData
implements java.io.Serializable extends UserORpublic class UserData extends
User implements java.io.SerializableThanks in advanceChristo


.