Why do we need empty serializable interface??

Question

I see almost all classes implementing serializable. Why cant we have this interace declared at the Object level itself??
abstract class Object implements Serialiazable{
Is there any reason for not having at the object level itself.???
And
Why they have not put method compareTo(Object o) inside Comparable interface???

Answer 1

Is there any reason for not having at the object level itself.???1. Not all objects are serializable.
Eg:Thread, OutputStream and its subclasses, and Socket are not serializable.
2. Serializable should be implemented judiciously.
Extracted from Effective Java: Programming Language Guide
Allowing a class's instances to be serialized can be as simple as adding the words
�implements Serializable� to its declaration. Because this is so easy to do, there is
a common misconception that serialization requires little effort on the part of the programmer.
The truth is far more complex. While the immediate cost to make a class serializable can be
negligible, the long-term costs are often substantial.There are numerous reasons. Please refer to the book.
>
And
Why they have not put method compareTo(Object o) inside Comparable interface???[http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Comparable.html#compareTo(T)]