Design Patterns

1. Design Patterns: Reusable Solutions to Common Problems

Design patterns capture solutions that have developed and evolved over time.

Capturing Design Expertise. Design patterns are not just theoretical concepts; they are practical solutions that have emerged from real-world software development experiences. They represent proven techniques for addressing recurring design challenges in object-oriented systems.

Four Essential Elements:

• Pattern Name: A handle for easy reference and communication.
• Problem: Describes when to apply the pattern.
• Solution: Describes the elements, relationships, and responsibilities.
• Consequences: Discusses the trade-offs and results of applying the pattern.

Benefits of Using Design Patterns. By leveraging design patterns, developers can avoid reinventing the wheel and instead build upon established, well-understood solutions. This leads to more flexible, elegant, and reusable object-oriented designs.

2. Creational Patterns: Abstracting Object Instantiation

Creational patterns concern the process of object creation.

Decoupling Object Creation. Creational patterns provide mechanisms to decouple the client code from the concrete classes being instantiated. This abstraction allows for greater flexibility in object creation, composition, and representation.

Class vs. Object Scope:

• Class creational patterns use inheritance to vary the class that's instantiated.
• Object creational patterns delegate instantiation to another object.

Key Creational Patterns: Abstract Factory, Builder, Factory Method, Prototype, and Singleton. These patterns offer different approaches to managing object creation, each with its own set of trade-offs and benefits.

3. Structural Patterns: Composing Classes and Objects

Structural patterns deal with the composition of classes or objects.

Building Complex Structures. Structural patterns focus on how classes and objects can be combined to form larger, more complex structures. These patterns address issues of composition, interface adaptation, and efficient object sharing.

Class vs. Object Scope:

• Class structural patterns use inheritance to compose interfaces or implementations.
• Object structural patterns use object composition to realize new functionality.

Key Structural Patterns: Adapter, Bridge, Composite, Decorator, Facade, Flyweight, and Proxy. These patterns provide solutions for creating flexible and maintainable object structures.

4. Behavioral Patterns: Defining Object Interaction and Responsibility

Behavioral patterns characterize the ways in which classes or objects interact and distribute responsibility.

Governing Object Communication. Behavioral patterns are concerned with algorithms and the assignment of responsibilities between objects. They describe not just patterns of objects or classes but also the patterns of communication between them.

Class vs. Object Scope:

• Class behavioral patterns use inheritance to distribute behavior between classes.
• Object behavioral patterns use object composition rather than inheritance.

Key Behavioral Patterns: Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template Method, and Visitor. These patterns offer solutions for managing complex control flow and object interactions.

5. The Composite Pattern: Representing Part-Whole Hierarchies

Composite lets clients treat individual objects and compositions of objects uniformly.

Recursive Composition. The Composite pattern enables the creation of hierarchical structures by composing objects into tree-like arrangements. This allows clients to treat individual objects and compositions of objects uniformly.

Key Elements:

• Component: Defines the interface for objects in the composition.
• Leaf: Represents leaf objects in the composition (no children).
• Composite: Defines behavior for components having children and stores child components.

Benefits: Simplifies client code, makes it easier to add new component types, and defines class hierarchies consisting of primitive objects and composite objects.

6. The Strategy Pattern: Encapsulating Algorithms for Flexibility

Strategy lets the algorithm vary independently from clients that use it.

Algorithm Encapsulation. The Strategy pattern encapsulates different algorithms within separate classes, making them interchangeable. This allows the algorithm to vary independently from the clients that use it.

Key Elements:

• Strategy: Declares an interface common to all supported algorithms.
• ConcreteStrategy: Implements the algorithm using the Strategy interface.
• Context: Is configured with a ConcreteStrategy object and maintains a reference to it.

Benefits: Provides an alternative to subclassing, eliminates conditional statements, and offers a choice of implementations.

7. The Observer Pattern: Establishing Dependencies for Automatic Updates

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Publish-Subscribe Mechanism. The Observer pattern defines a one-to-many dependency between objects, where a subject notifies its observers of any changes in its state. This allows for automatic updates and maintains consistency between related objects.

Key Elements:

• Subject: Knows its observers and provides an interface for attaching and detaching them.
• Observer: Defines an updating interface for objects that should be notified of changes in a subject.
• ConcreteSubject: Stores state of interest to ConcreteObserver objects and sends a notification to its observers when its state changes.
• ConcreteObserver: Maintains a reference to a ConcreteSubject object, stores state that should stay consistent with the subject's, and implements the Observer updating interface.

Benefits: Abstract coupling between Subject and Observer, support for broadcast communication, and the ability to vary subjects and observers independently.

8. The Power of Loose Coupling and Abstraction

Program to an interface, not an implementation.

Reducing Dependencies. Loose coupling and abstraction are recurring themes throughout the design patterns. By minimizing dependencies between classes and programming to interfaces rather than concrete implementations, systems become more flexible, reusable, and maintainable.

Benefits of Loose Coupling:

• Increased reusability of classes in isolation.
• Easier to learn, port, modify, and extend a system.
• Reduced risk of cascading changes when one class is modified.

Techniques for Achieving Loose Coupling: Abstract coupling, layering, and encapsulating the concept that varies. These techniques are applied in various design patterns to promote loosely coupled systems.

9. Selecting and Applying Design Patterns Effectively

A design pattern should only be applied when the flexibility it affords is actually needed.

Context-Specific Application. Design patterns should not be applied indiscriminately. They are tools to be used when the specific problem they address arises. Applying a pattern without a clear understanding of its benefits and liabilities can lead to unnecessary complexity and reduced performance.

Approaches to Finding the Right Pattern:

• Consider how design patterns solve design problems.
• Scan Intent sections.
• Study how patterns interrelate.
• Study patterns of like purpose.
• Examine a cause of redesign.
• Consider what should be variable in your design.

Step-by-Step Approach to Applying a Design Pattern: Read the pattern, study the structure, look at the sample code, choose meaningful names, define the classes, define application-specific names for operations, and implement the operations.

10. Design Patterns as a Common Language and Learning Tool

Design patterns provide a common vocabulary for designers to use to communicate, document, and explore design alternatives.

Enhancing Communication. Design patterns provide a shared vocabulary for discussing and documenting software designs. This allows developers to communicate more effectively and understand existing systems more easily.

Improving Design Skills. Learning design patterns can accelerate the development of expertise in object-oriented design. They provide solutions to common problems and guide designers in making informed decisions.

Complementing Existing Methods. Design patterns are not a replacement for design methods but rather a valuable adjunct. They provide a way to describe more of the "why" of a design and not just record the results of your decisions.

Last updated: February 21, 2025