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Learn: Application Design Patterns

Concept-focused guide for Application Design Patterns (no answers revealed).

~9 min read

Overview

Welcome! In this deep-dive vlog transcript, we’ll unlock the principles behind some of the most powerful application design patterns: creational, behavioral, and structural patterns. You’ll learn to recognize, analyze, and apply key patterns in software architecture—enabling you to write code that’s more scalable, maintainable, and flexible. Expect a mix of clear definitions, practical reasoning strategies, and illustrative scenarios to help you confidently tackle pattern-centric challenges.

Concept-by-Concept Deep Dive

Isolating Object Creation: Factory, Abstract Factory, and Builder Patterns

What It Is:
Creational patterns focus on how objects are created. They help manage object instantiation, decouple clients from specific classes, and encapsulate the logic of creation so your code is more flexible and less dependent on concrete implementations.

Factory Method

  • Purpose: Delegates instantiation to subclasses, allowing code to use interfaces rather than concrete types.
  • Components: Creator (defines factory method), ConcreteCreator (implements factory method), Product (interface), ConcreteProduct.
  • Recipe: When you have a superclass and want to defer the instantiation of objects to subclasses, use a factory method. This is especially useful when the client code shouldn’t know which specific class it needs.
  • Misconceptions: Don’t confuse with simple constructors; factory methods enable substituting different product types without changing client code.

Abstract Factory

  • Purpose: Produces families of related objects (products), ensuring that products from the same family are used together.
  • Components: AbstractFactory (interface), ConcreteFactory, AbstractProduct, ConcreteProduct.
  • Recipe: Use when you need to create sets of related objects (e.g., UI components for different themes) and want to guarantee their compatibility.
  • Misconceptions: Abstract Factory is not about creating one object but about creating related groups.

Builder

  • Purpose: Constructs a complex object step by step, allowing different representations.
  • Components: Builder (interface), ConcreteBuilder, Director, Product.
  • Recipe: Ideal when an object requires numerous optional parameters or has a complex construction process. The builder pattern gives you fine-grained control.
  • Misconceptions: Not just for optional parameters—also for complex assembly processes.

Dynamic Object Behavior: Decorator, Proxy, and State Patterns

What It Is:
Structural and behavioral patterns enable objects to acquire new functionality or change their behavior dynamically without modifying their code. This means you can extend or modify an object’s behavior at runtime.

Decorator

  • Purpose: Adds responsibilities to objects dynamically, offering an alternative to subclassing for extending functionality.
  • Components: Component (interface), ConcreteComponent, Decorator (wraps a component), ConcreteDecorator.
  • Recipe: Wrap the original object with one or more decorators to add new behavior or state.
  • Misconceptions: Not just for UI elements—any scenario where dynamic extension is needed.

Proxy

  • Purpose: Controls access to another object, adding functionality such as security, logging, or lazy instantiation.
  • Components: Subject (interface), RealSubject, Proxy.
  • Recipe: Implement a proxy that has the same interface as the real subject and delegates requests, adding extra logic as needed.
  • Misconceptions: Proxy is not always about remote access; it can also be for protection, logging, or caching.

State

  • Purpose: Allows an object to change its behavior when its internal state changes, appearing as if it changed its class.
  • Components: Context, State (interface), ConcreteState.
  • Recipe: The context holds a reference to a state object, and behavior is delegated to the current state’s implementation.
  • Misconceptions: State is not simply a switch statement—it's about encapsulating behavior per state.

Managing Complex Structures: Composite, Facade, Flyweight, and Bridge Patterns

What It Is:
Structural patterns help organize classes and objects into larger structures, making them easier to work with and maintain.

Composite

  • Purpose: Composes objects into tree structures to represent part-whole hierarchies. Clients treat individual objects and compositions uniformly.
  • Components: Component (interface), Leaf, Composite.
  • Recipe: Both leaf and composite implement the same interface, so clients can interact with a single object or a group in the same way.
  • Misconceptions: Composite is not just for graphics; it's for any hierarchical, recursive structure.

Facade

  • Purpose: Provides a simplified interface to a complex subsystem, reducing dependencies.
  • Components: Facade class, subsystem classes.
  • Recipe: The facade class delegates calls to appropriate subsystem classes, hiding complexity from the client.
  • Misconceptions: Facade does not add functionality; it just simplifies access.

Flyweight

  • Purpose: Minimizes memory use by sharing as much data as possible with similar objects.
  • Components: Flyweight (interface), ConcreteFlyweight (shared), UnsharedConcreteFlyweight, FlyweightFactory.
  • Recipe: Separate intrinsic (shared) from extrinsic (unique) state. Clients supply extrinsic state when using flyweights.
  • Misconceptions: Only apply when you have a huge number of similar objects.

Bridge

  • Purpose: Decouples abstraction from implementation, allowing them to vary independently.
  • Components: Abstraction, RefinedAbstraction, Implementor (interface), ConcreteImplementor.
  • Recipe: Abstraction holds a reference to the implementor interface, delegating implementation-specific work.
  • Misconceptions: Bridge is not just about interfaces; it’s about separating what you do from how you do it.

Encapsulating Behavior: Command, Strategy, Interpreter, Template Method, Visitor, and Iterator Patterns

What It Is:
Behavioral patterns delegate responsibilities between objects, encapsulate requests, and enable flexible communication and control flows.

Command

  • Purpose: Encapsulates a request as an object, allowing for parameterization, queuing, undo/redo, and logging.
  • Components: Command (interface), ConcreteCommand, Receiver, Invoker, Client.
  • Recipe: Actions are implemented as command objects, which can be executed, undone, or stored.
  • Misconceptions: Command is not only about GUI buttons; it's general for any action encapsulation.

Strategy

  • Purpose: Defines a family of algorithms, encapsulates each, and makes them interchangeable.
  • Components: Strategy (interface), ConcreteStrategy, Context.
  • Recipe: The context delegates algorithmic work to the strategy interface, allowing it to switch strategies at runtime.
  • Misconceptions: Strategy is not about subclassing; it’s about composition.

Interpreter

  • Purpose: Defines a grammar and an interpreter for processing sentences in the language.
  • Components: AbstractExpression, TerminalExpression, NonTerminalExpression, Context.
  • Recipe: Build an object structure that represents the grammar, then interpret expressions recursively.
  • Misconceptions: Interpreter is for simple, well-defined grammars—not for complex language parsing.

Template Method

  • Purpose: Defines the skeleton of an algorithm in a method, deferring some steps to subclasses.
  • Components: AbstractClass (with template method), ConcreteClass (overrides steps).
  • Recipe: Base class implements the algorithm structure, subclasses override specific steps.
  • Misconceptions: Template method is not just code reuse; it’s about controlling algorithm steps.

Visitor

  • Purpose: Separates algorithms from the objects they operate on, allowing new operations without changing classes.
  • Components: Visitor (interface), ConcreteVisitor, Element (interface), ConcreteElement.
  • Recipe: Visitor interface declares visit methods for each element type; elements accept visitors.
  • Misconceptions: Visitor adds operations, not state or structure.

Iterator

  • Purpose: Provides a way to access elements of a collection sequentially without exposing its underlying representation.
  • Components: Iterator (interface), ConcreteIterator, Aggregate/Collection.
  • Recipe: The iterator maintains position and exposes next() and hasNext() methods.
  • Misconceptions: Iterator is not just for arrays; it works with any collection or composite structure.

Object Lifetime Control: Singleton and Prototype Patterns

What It Is:
Some patterns control the number and identity of objects or allow for efficient object creation via cloning.

Singleton

  • Purpose: Ensures a class has only one instance and provides a global point of access.
  • Components: Private constructor, static instance, public static accessor.
  • Recipe: Make constructor private, hold the single instance as a private static field, and provide a public static method for access.
  • Misconceptions: In multithreaded environments, naive singletons can cause bugs—use thread-safe initialization.

Prototype

  • Purpose: Creates new objects by copying existing instances (cloning), rather than instantiating new ones.
  • Components: Prototype (interface), ConcretePrototype (implements clone), Client.
  • Recipe: The client requests a clone of a prototype, which returns a new instance with the same state.
  • Misconceptions: Prototype is not just for copying data—it's for creating new instances efficiently, especially when construction is expensive.

Worked Examples (generic)

  1. Factory Method Example:

    • Suppose you’re building a notification system supporting SMS and Email. You define a Notification interface and subclasses SMSNotification and EmailNotification. A NotificationFactory class decides which notification to create based on a parameter. The client asks the factory for a notification without knowing the concrete class.
    • Reasoning: The factory method encapsulates the logic for deciding which subclass to instantiate.

  2. Decorator Example:

    • You have a TextView class. To add scrollbars and borders without modifying TextView, you create ScrollableDecorator and BorderedDecorator, both implementing the same interface as TextView. You wrap a TextView with these decorators at runtime.
    • Reasoning: Each decorator adds new behavior, and you can combine them as needed.

  3. Composite Example:

    • In a graphics editor, you have a Shape interface. Circle and Rectangle are leaves, while Group is a composite that can contain shapes and other groups. Methods like draw() are called on both individual shapes and groups.
    • Reasoning: Clients use the same interface for both simple and complex (composed) objects.

  4. Strategy Example:

    • A sorting utility provides a SortStrategy interface with implementations for QuickSort, MergeSort, etc. A Sorter class holds a reference to a SortStrategy, allowing the algorithm to be selected at runtime.
    • Reasoning: The sorting algorithm is interchangeable, and the client is decoupled from specific implementations.

Common Pitfalls and Fixes

  • Confusing Patterns with Similar Names:
    Factory vs. Abstract Factory vs. Builder—review the intent and structure of each to pick the right one.

  • Overusing Singleton:
    Singletons can introduce hidden dependencies and threading issues; ensure thread safety and testability.

  • Decorator vs. Inheritance:
    Decorators add behavior at runtime, unlike inheritance, which is static—prefer decorators when flexibility is needed.

  • Misapplying Composite:
    Remember composite is about part-whole hierarchies, not just grouping objects.

  • Proxy vs. Decorator:
    Proxy controls access; decorator adds behavior. Don’t use proxy for dynamic extension.

  • Visitor Pattern Misuse:
    Only use visitor when you need to add operations across many unrelated classes—otherwise, it adds unnecessary complexity.

  • Ignoring Thread Safety in Patterns:
    Patterns like Singleton and Flyweight require careful handling of shared resources in multithreaded environments.

Summary

  • Creational patterns (Factory, Abstract Factory, Builder, Singleton, Prototype) help manage object creation and decouple clients from concrete classes.
  • Structural patterns (Decorator, Proxy, Composite, Facade, Flyweight, Bridge) organize relationships between objects and classes for flexibility and maintainability.
  • Behavioral patterns (Command, Strategy, Interpreter, Template Method, Visitor, Iterator, State) encapsulate control flow, algorithm families, and object interactions.
  • Recognize patterns in context by their intent (what problem they solve) and structure (how they organize classes/objects).
  • Avoid common pitfalls by focusing on the pattern’s purpose, not just its structure—always ask “what problem am I solving?” before choosing a pattern.