Design Patterns - J2EE and GoF Patterns

The Decorator pattern attaches additional responsibilities to an object dynamically. It wraps a component (both implement the same interface) to add behavior without modifying the original class. Option A describes Bridge pattern (decoupling abstraction from implementation), Option C describes Composite pattern (tree structures of objects), and Option D describes Facade pattern (simplified interface to a complex subsystem).

The Facade pattern provides a simplified interface to a complex subsystem. One class (the facade) offers a unified interface that delegates to multiple underlying classes, hiding their complexity. Option A describes Bridge pattern, Option C describes Decorator pattern, and Option B is too vague but could describe Proxy or Factory patterns.

Creational patterns deal with object creation mechanisms. Singleton ensures only one instance exists, Builder constructs complex objects step-by-step, and Factory Method lets subclasses decide which class to instantiate. The other options list Structural patterns (A), Structural patterns (C), and Behavioral patterns (D).

A Proxy pattern acts as a surrogate or placeholder for another object to control access to it. Proxies typically implement the same interface as the real subject, allowing them to be used interchangeably. They can add lazy initialization, access control, or logging without changing the real subject's code.

java.io.BufferedInputStream wraps another InputStream (like FileInputStream) to add buffering functionality. Both implement the InputStream interface, and BufferedInputStream adds behavior (buffering for performance) without changing the underlying stream - this is the Decorator pattern in action. java.util.List and java.lang.String are not examples of Decorator.

The Template Method pattern defines the skeleton of an algorithm in a base class but defers some steps to subclasses. This allows the invariant parts (unchanging steps) to be implemented once in the base class while letting subclasses customize the variable parts. Strategy pattern defines a family of algorithms, and Observer pattern defines a subscription mechanism.

Java's AWT/Swing event model uses the Observer pattern (also called Listener pattern in this context). Components (subjects) maintain lists of listeners (observers) and notify them when events occur (like button clicks). Listeners implement event listener interfaces and register with components to receive notifications. Strategy and Template patterns don't describe this publish-subscribe relationship.

The Composite pattern allows you to compose objects into tree structures to represent part-whole hierarchies. It lets clients treat individual objects and compositions uniformly. 'Recursive composition' refers to composites that can contain other composites (nested structures), building complex objects from simpler ones using the same interface at every level.

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. The algorithm can vary independently from the clients that use it, and different strategies can be selected at runtime. This is exactly 'dynamic selection of algorithm implementations'. Facade and Adapter are structural patterns concerned with interfaces, not algorithm selection.

The Factory Pattern is specifically designed to defer object instantiation to subclasses or factory methods, allowing the decision of which compatible class to instantiate to be made at runtime based on conditions or configuration. Singleton ensures only one instance exists, Facade provides a simplified interface to a complex subsystem, and Adapter makes incompatible interfaces work together - none of these primarily address runtime instantiation decisions.

For a Singleton pattern, the getInstance() method must be static to be accessible without an object reference, and typically public to allow access. However, the code shown creates a new Connection() on every call, which violates Singleton principles - it should return the same single instance. A proper Singleton needs a private constructor and a static field holding the single instance.

The Flyweight Pattern is specifically designed to minimize memory usage by sharing intrinsic state (common data like colors) among multiple objects. For 1000 circles with only 6 colors, Flyweight would share the 6 color objects rather than storing color with each circle, dramatically reducing memory. Facade simplifies interfaces, Decorator adds responsibilities dynamically, and Adapter converts interfaces - none optimize memory like Flyweight.

The Interpreter Pattern is designed to parse and evaluate sentences in a language according to a grammar. It defines a representation for the grammar and an interpreter that uses that representation to interpret sentences. Command encapsulates requests, Observer defines notification updates, and Iterator traverses collections - none handle parsing and translation like Interpreter.

The Iterator Pattern provides a way to access elements of an aggregate object sequentially without exposing its underlying representation. It decouples traversal from the aggregate structure. Command encapsulates requests, Interpreter handles grammar evaluation, and Mediator centralizes communication - none provide sequential traversal like Iterator.

Data Access Object (DAO) Pattern abstracts and encapsulates access to data sources, providing a uniform interface to different backends like legacy systems, B2B interfaces, or LDAP. It hides implementation details of data source interactions. MVC separates concerns, Business Delegate reduces coupling between tiers, and Service Locator looks up resources - none abstract heterogeneous data access like DAO.