Design patterns are typical solutions to common problems in software design. They represent best practices that a programmer can use to solve common problems when designing an application or system. This guide covers some of the most important design patterns, complete with detailed examples and visual diagrams to help you understand each pattern.
1. Observer Pattern
Definition
The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Example
Consider a newspaper system where multiple subscribers (observers) receive updates when a new edition (subject) is published. Each subscriber is notified when the newspaper releases a new edition, allowing them to read the latest news.
Implementation
- Subject: Maintains a list of observers and notifies them of any state changes.
- Observer: Defines an updating interface for objects that should be notified of changes in the subject.
- ConcreteSubject: Implements the subject interface and stores the state of interest to the observers.
- ConcreteObserver: Implements the observer updating interface to keep its state consistent with the subject’s.
2. Strategy Pattern
Definition
The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. This pattern allows the algorithm to vary independently from clients that use it.
Example
Imagine a navigation app that can use different routing algorithms (strategies) such as shortest path, scenic route, or fastest route based on user preference. The app can switch between these algorithms without changing the code that uses them.
Implementation
- Strategy: Declares an interface common to all supported algorithms.
- ConcreteStrategy: Implements the algorithm using the Strategy interface.
- Context: Uses a ConcreteStrategy to execute the algorithm.
3. State Pattern
Definition
The State Pattern allows an object to alter its behavior when its internal state changes, effectively encapsulating state-specific logic into separate classes.
Example
Consider a video player that has different states (playing, paused, stopped). The behavior of the play button changes depending on the current state of the player. For instance, if the player is in a paused state, pressing play will resume the video.
Implementation
- State: Defines an interface for encapsulating the behavior associated with a particular state.
- ConcreteState: Each subclass implements a behavior associated with a state of the Context.
- Context: Maintains an instance of a ConcreteState subclass that defines the current state.
4. Adapter Pattern
Definition
The Adapter Pattern allows incompatible interfaces to work together by acting as a bridge that converts one interface into another expected by the client.
Example
Think of a card reader that reads different memory cards and adapts them to a USB interface. The card reader acts as an adapter, allowing the computer (client) to communicate with various memory cards (adaptees) through a standard USB interface.
Implementation
- Target: Defines the domain-specific interface that Client uses.
- Adapter: Adapts the interface of Adaptee to the Target interface.
- Adaptee: Defines an existing interface that needs adapting.
- Client: Collaborates with objects conforming to the Target interface.
5. Bridge Pattern
Definition
The Bridge Pattern decouples an abstraction from its implementation, allowing them to vary independently by placing them in separate class hierarchies.
Example
Consider a graphics application that can draw shapes like circles and squares in different ways (e.g., using vector graphics or raster graphics). The shape and rendering method can vary independently, so the Bridge Pattern is used to separate these concerns.
Implementation
- Abstraction: Defines the abstraction’s interface and maintains a reference to an object of type Implementor.
- RefinedAbstraction: Extends the interface defined by Abstraction.
- Implementor: Defines the interface for implementation classes.
- ConcreteImplementor: Implements the Implementor interface.
6. Composite Pattern
Definition
The Composite Pattern allows individual objects and compositions of objects to be treated uniformly by organizing objects into tree structures to represent part-whole hierarchies.
Example
Consider a graphic design application where drawings consist of shapes (like circles and rectangles) that can contain other shapes. Each shape can be manipulated individually or as part of a larger composition.
Implementation
- Component: Declares the interface for objects in the composition.
- Leaf: Represents leaf objects in the composition and implements the Component interface.
- Composite: Represents a node in the composition and implements the Component interface, storing child components.
7. Facade Pattern
Definition
The Facade Pattern provides a simplified, unified interface to a complex subsystem, making it easier to use and understand.
Example
Consider a home entertainment system with various components like a TV, DVD player, and sound system. A Facade can provide a simple interface to control all these components, such as “watch movie” which turns on the TV, starts the DVD player, and sets up the sound system.
Implementation
- Facade: Provides a unified interface to a set of interfaces in a subsystem.
- Subsystem Classes: Implement subsystem functionality and handle work assigned by the Facade object.
8. Flyweight Pattern
Definition
The Flyweight Pattern minimizes memory usage by sharing as much data as possible with other similar objects, using a fine-grained instance for efficient resource management.
Example
Consider a text editor where each character is a flyweight object. Instead of creating a new object for every character in the document, the editor reuses existing character objects, saving memory and improving performance.
Implementation
- Flyweight: Declares an interface through which flyweights can receive and act on extrinsic state.
- ConcreteFlyweight: Implements the Flyweight interface and adds storage for intrinsic state.
FlyweightFactory: Creates and manages flyweight objects
9. Proxy Pattern
Definition
The Proxy Pattern provides a surrogate or placeholder for another object to control access to it, enhancing functionality like lazy initialization, access control, or logging.
Example
Consider a virtual proxy for loading large images. The proxy represents the image and loads it only when it’s actually needed, saving memory and improving performance.
Implementation
- Subject: Defines the common interface for RealSubject and Proxy.
- RealSubject: Represents the real object that the proxy controls access to.
- Proxy: Maintains a reference to the RealSubject and controls access to it.
1. Factory Pattern
Definition
The Factory Pattern provides an interface for creating objects in a superclass, but allows subclasses to alter the type of objects that will be created.
Example
Consider a logistics application that needs to create different types of transport (e.g., trucks, ships). The factory pattern allows the creation of transport objects without specifying the exact class of the object that will be created.
Implementation
- Product: Declares the interface for objects the factory method creates.
- ConcreteProduct: Implements the Product interface.
- Creator: Declares the factory method, which returns an object of type Product.
- ConcreteCreator: Overrides the factory method to return an instance of a ConcreteProduct.
2. Abstract Factory Pattern
Definition
The Abstract Factory Pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes.
Example
Consider a UI toolkit that supports multiple themes (e.g., dark theme, light theme). Each theme requires a set of related UI components such as buttons, scroll bars, and text fields. The abstract factory pattern allows the creation of families of related UI components without specifying their concrete classes.
Implementation
- AbstractFactory: Declares an interface for operations that create abstract product objects.
- ConcreteFactory: Implements the operations to create concrete product objects.
- AbstractProduct: Declares an interface for a type of product object.
- ConcreteProduct: Defines a product object to be created by the corresponding concrete factory.
- Client: Uses only interfaces declared by AbstractFactory and AbstractProduct classes.
3. Builder Pattern
Definition
The Builder Pattern constructs a complex object step by step, allowing for greater control over the construction process and enabling different representations of the object.
Example
Consider a meal ordering system where you can create a meal with various components like a burger, fries, and a drink. The builder pattern allows the creation of different types of meals by combining these components in different ways.
Implementation
- Builder: Specifies an abstract interface for creating parts of a Product object.
- ConcreteBuilder: Constructs and assembles parts of the product by implementing the Builder interface.
- Director: Constructs an object using the Builder interface.
- Product: Represents the complex object being built.
4. Prototype Pattern
Definition
The Prototype Pattern creates new objects by copying an existing object, known as the prototype, rather than creating instances from scratch.
Example
Consider a document editor where you can duplicate documents. The prototype pattern allows creating new documents by copying an existing document, thus saving the overhead of creating objects from scratch.
Implementation
- Prototype: Declares an interface for cloning itself.
- ConcretePrototype: Implements the operation for cloning itself.
- Client: Creates a new object by asking a prototype to clone itself.
5. Singleton Pattern
Definition
The Singleton Pattern ensures that a class has only one instance and provides a global point of access to that instance.
Example
Consider a logging class that needs to be shared across different parts of an application. The singleton pattern ensures that only one instance of the logging class is created and provides a global point of access to it.
Implementation
- Singleton: Declares a static method that returns the instance of the singleton class.
6. Chain of Responsibility Pattern
Definition
The Chain of Responsibility Pattern passes a request along a chain of handlers, where each handler decides either to process the request or to pass it to the next handler in the chain.
Example
Consider a help desk application where a support request can be handled by different levels of support staff. The chain of responsibility pattern allows the request to be passed along the chain until it reaches a handler that can process it.
Implementation
- Handler: Declares an interface for handling requests.
- ConcreteHandler: Handles requests it is responsible for and can access its successor.
- Client: Initiates the request to a ConcreteHandler object.
7. Command Pattern
Definition
The Command Pattern encapsulates a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations, as well as support for undoable operations.
Example
Consider a text editor where you can perform and undo various text operations like typing, deleting, and formatting. The command pattern encapsulates each operation as an object, allowing for flexible command execution and undo functionality.
Implementation
- Command: Declares an interface for executing an operation.
- ConcreteCommand: Implements the execute method by invoking the corresponding operation(s) on Receiver.
- Invoker: Asks the command to carry out the request.
- Receiver: Knows how to perform the operations associated with carrying out a request.
8. Iterator Pattern
Definition
The Iterator Pattern provides a way to access the elements of a collection sequentially without exposing the underlying representation.
Example
Consider a social media application where you need to iterate over a list of user posts. The iterator pattern allows sequential access to the posts without exposing the underlying list structure.
Implementation
- Iterator: Defines an interface for accessing and traversing elements.
- ConcreteIterator: Implements the Iterator interface and keeps track of the current position in the traversal.
- Aggregate: Defines an interface for creating an iterator object.
- ConcreteAggregate: Implements the Aggregate interface and returns an instance of ConcreteIterator.
Conclusion
These design patterns are essential tools for software developers, providing proven solutions to common problems. Understanding and implementing these patterns can significantly improve your code’s structure, flexibility, and maintainability. By using visual diagrams and detailed examples, this guide aims to make these patterns more accessible and easier to understand.
