If we have more than two classes with same method names but 
their implementation is different.we can go for interface.
Example: JDBC API Interfaces. Every data base vender 
implements these interfaces with thier implementation code.

If we want to use some common method implementaions for 
multiple classes along with their own implementation we can 
go for abstract class.

Example:Servlet API:
HttpServlet is an abstract class in which init() is 
implemented used by every servlet that extends.Every 
servlet implement their own service method.