yes it is very much compulsory to have a one abstract mthod 
in abstract class
otherwise will be no longer abstract

No.  It is not compulsory that an abstract class
should have atleast one abstract method.

One can declare a class as abstract still with full
implementation of methods.  This avoids the instantiation
the class

No,It is not mandatory, but sub class of abstract class 
should be override by any one of the method from abstract 
class .

Ex:i)  javax.servlet.http.HttpServlet
   ii) org.apache.struts.actions.DispatchAction, etc.,

yes,whenever a class is defined as an abstract it should 
have atleast one method as an abstract

Yes, it is compulsory to have atleast one abstract method 
in abstract class

abstract class can have methods without abstract methods.
if someone dont konw the proper answer then please dont 
give the answer.
this is not the proper way.