Implementing Serializable interface signifies that the 
object can be persisted.  Otherwise, when you try to
use writeObject, it will throw 
java.io.NotSerializableException

well if we dont use Serializable interface which is a 
marker interface, then the object which we want to 
serialize and use writeObject() method then compiler would 
throw exception as if other object which r related to that 
object if not serialized.
So. to overcome this we have to use Seializable interface.

Ya that answers are expected but...
If we open ObjectOutputStream.java (which is used for
serialization) then we see the following code :

............................
writeObject0(Object obj, boolean unshared){
.............
 } else if (cl.isArray()) {
		writeArray(obj, desc, unshared);
	    } else if (obj instanceof Serializable) {
		writeOrdinaryObject(obj, desc, unshared);
	    } else {
		throw new NotSerializableException(cl.getName());
	    }
................

If we remove the checking instanceof (& recompile)  or if we
override the writeObject0 method then 
Will it still check whethere the class has implemented
serilizable ?