J2EE Design Patterns and Enterprise Beans

The Data Access Object pattern is ideal when accessing multiple heterogeneous data sources. Each data source type (relational database, LDAP, flat files) gets its own DAO implementation exposing a common interface. The application code works with the DAO interface, remaining unaware of underlying storage differences. Business Delegate reduces coupling, Factory creates objects, and Builder constructs complex objects; none address multi-source data access.

The Factory pattern creates objects without specifying their exact class. When you have a superclass and multiple subclasses, and need to instantiate the appropriate subclass based on input data, a Factory method decides which class to instantiate and returns it. Singleton ensures one instance, Builder constructs complex objects step-by-step, and Prototype clones existing objects; none solve the conditional instantiation problem.

The Builder pattern separates complex object construction from its representation. It constructs objects step-by-step through a fluent interface, allowing different representations from the same construction process. Factory and Abstract Factory create objects in one step, Singleton restricts instantiation; none handle stepwise construction of complex objects.

The Digital Office application uses stateless session beans. Stateless beans are more efficient for stateless operations as they don't maintain conversational state with clients and can be pooled. Stateful beans would be used only if client-specific state needs to be maintained across method calls.

The validate() method in ActionForm (Struts framework) is used for form validation. This method is called automatically when the form is submitted and returns ActionErrors containing validation failure messages.