Master Design Patterns
Through Interactive Learning

The Problem: Sometimes you need exactly one instance of a class - like a database connection, a logger, or application settings.

The Solution: The Singleton pattern ensures only one instance exists and provides a global way to access it.

The Problem: You need to create objects, but you don't know exactly which type until runtime. Hard-coding object creation makes your code rigid and difficult to extend.

The Solution: The Factory pattern provides a way to create objects without specifying their exact class, making your code more flexible and easier to maintain.

The Problem: When you need to create objects with many optional parameters or complex construction logic, constructors become unwieldy and confusing.

The Solution: The Builder pattern lets you construct complex objects step by step with a fluent interface, making creation clear and flexible.

The Problem: When you need to create families of related objects that must be used together, but you want to ensure compatibility within each family.

The Solution: Use an abstract factory interface with concrete factories that produce families of compatible objects.

The Problem: When creating objects is expensive or complex, and you want to avoid the cost of initialization by reusing existing objects.

The Solution: Create new objects by cloning existing prototypes, allowing for efficient object creation with customizable properties.

The Problem: You have two classes with incompatible interfaces that need to work together, like trying to plug a European device into an American outlet.

The Solution: The Adapter pattern acts as a bridge between incompatible interfaces, allowing them to work together without changing their existing code.

The Problem: You want to add new functionality to objects without modifying their structure or creating many subclasses for every possible combination.

The Solution: The Decorator pattern lets you wrap objects with new behaviors dynamically, like adding layers to a cake or options to a car.

The Problem: You have a complex subsystem with many classes and methods, making it difficult for clients to understand and use effectively.

The Solution: The Facade pattern provides a simple, unified interface to a complex subsystem, like a TV remote that hides the complexity of the electronics inside.

The Problem: When you have multiple implementations of an abstraction and want to avoid a massive class hierarchy with every possible combination.

The Solution: Separate the abstraction from its implementation, allowing both to vary independently through composition.

The Problem: When you need to represent part-whole hierarchies where clients should treat individual objects and compositions uniformly.

The Solution: Create a tree structure where both individual objects and compositions implement the same interface.

The Problem: When you need to control access to an object, add functionality, or delay expensive operations without changing the client code.

The Solution: Create a proxy object that implements the same interface as the original object, controlling access and adding behavior.

The Problem: When you need to support large numbers of similar objects efficiently, but naive approach would consume too much memory.

The Solution: Share common intrinsic state between objects while keeping variable extrinsic state separate.

The Problem: One object changes, and you need to notify multiple other objects about this change. Without a good system, you end up with tightly coupled code that's hard to maintain.

The Solution: The Observer pattern lets objects "subscribe" to events and automatically get notified when something changes, like a newsletter subscription system.

The Problem: You have multiple ways to do the same thing (like different sorting algorithms), and you want to choose which one to use at runtime without changing your main code.

The Solution: The Strategy pattern lets you swap algorithms like changing tools in a toolbox. You define different strategies and choose which one to use based on the situation.

The Problem: You want to decouple objects that invoke operations from objects that perform them, and you need features like undo/redo or queuing operations.

The Solution: The Command pattern turns requests into objects, allowing you to parameterize, queue, log, and undo operations, like having a universal remote for any device.

The Problem: You want to process requests through a series of handlers, but you don't want to couple the sender to specific receivers.

The Solution: Create a chain of handler objects that can process requests in sequence, with each handler deciding whether to process or pass along the request.

The Problem: You need to traverse a collection of objects without exposing its underlying structure or implementation details.

The Solution: Provide a standard way to access elements sequentially without exposing the collection's internal representation.

The Problem: An object needs to change its behavior when its internal state changes, and you want to avoid large conditional statements.

The Solution: Create separate state classes that encapsulate state-specific behavior, allowing objects to change behavior dynamically.

The Problem: You have an algorithm with a fixed sequence of steps, but some steps need to be customized for different situations.

The Solution: Define the algorithm's skeleton in a base class, letting subclasses override specific steps without changing the overall structure.