Design Pattern Cheat Sheet for Developers

Design patterns are proven, reusable solutions to common software design problems. They provide developers with a structured approach to building systems that are flexible, scalable, and maintainable. 

Design Patterns Cheat Sheet: Full Guide

Step 1: Design Patterns Basics

What is a Design Pattern?

A design pattern is a reusable, well-tested solution to a common problem in software design. It offers a standard way to address common problems in a specific area. This approach encourages code that is easy to maintain, extend, and reuse. It serves as a blueprint for organizing code, creating objects, or managing interactions, helping developers write clean, maintainable, and scalable software while following best practices.

Key Properties of Design Patterns:

• Reusability: Design patterns provide solutions that can be reused across projects.
• Flexibility: They are easily extended or modified for new requirements.
• Abstraction: They focus on high-level design concepts, not implementation.
• Maintainability: Makes code easier to read, update, and manage.
• Scalability: Design Pattern supports growth in size and complexity of software.

How do Design Patterns Work?

To understand how software architecture design patterns actually work, let’s go through the process step by step:

• Identify the Problem: Find recurring issues like tight coupling, poor reuse, or scaling challenges.
• Choose the Right Pattern: Select a design pattern that best addresses the problem.
• Understand the Structure: Learn the pattern’s components and how they interact.
• Apply to Code: Implement the pattern to organize and simplify your code.
• Test and Refine: Verify functionality and tweak as needed while preserving pattern benefits.

Step 2: Design Patterns Classification

Design patterns are classified into three main types based on purpose. Each pattern has a structure including intent, motivation, applicability, structure (UML), participants, collaborations, consequences, and implementation

1. Creational Patterns

Creational Patterns focus on object creation mechanisms rather than using direct constructors.

• They ensure flexibility and control over how objects are instantiated.
• They help to reuse existing objects or create new ones efficiently.
• Examples include Singleton, Factory Method, Abstract Factory, Builder, and Prototype.
• They are ideal for scenarios where object creation is complex, resource-intensive, or needs customization.

2. Structural Patterns

Structural Patterns focus on object composition and relationships to form larger structures.

• They ensure maintainable and reusable code by defining clear relationships between objects.
• They help simplify complex architectures and reduce system complexity.
• Examples include Adapter, Bridge, Composite, Decorator, Facade, Flyweight, and Proxy.
• They are ideal for scenarios where objects need to work together in flexible and extensible ways.

3. Behavioral Patterns

Behavioral Patterns focus on communication, algorithms, and assignment of responsibilities among objects.

• They ensure clear, predictable, and loosely coupled interactions between classes.
• They help manage dynamic behavior and event-driven workflows efficiently.
• Examples include Observer, Strategy, Command, Iterator, Mediator, State, Template Method, Visitor, Memento, and Chain of Responsibility.
• They are ideal for scenarios where object behavior is complex, responsibilities need clear assignment, or algorithms must be easily modified.

Step 3: Creational Design Patterns

Creational design patterns are primarily concerned with how objects are created. They help simplify object creation, ensure flexibility, and promote reusability.

Creational Design Pattern Examples:

• Singleton – Ensures only one instance of a class exists and provides a global access point to it.
• Factory Method – Defines an interface for creating objects but lets subclasses decide which object to instantiate.
• Abstract Factory – Provides an interface to create families of related objects without specifying their concrete classes.
• Builder – Separates object construction from its representation, allowing step-by-step creation of complex objects.
• Prototype – Creates new objects by cloning an existing object (prototype) instead of instantiating from scratch.

• When object creation is complex or involves multiple steps.
• When different representations of objects are required.
• When system needs controlled instantiation (e.g., only one instance allowed).
• When you want to decouple the creation of objects from their usage.

Example 1: Singleton Pattern

1. The constructor is made private to prevent direct instantiation.
2. A static method provides access to the single instance.
3. Ensures that no more than one object of the class exists.

• Logging systems where a single log manager is used throughout the application.
• Database connection pools where only one shared connection manager is needed.

Example Code (Java):

public class Singleton {
    private static Singleton instance;
    
    private Singleton() {} // private constructor
    
    public static Singleton getInstance() {
        if(instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

Example 2: Factory Method Pattern

1. The factory class contains a method to create objects.
2. The client code calls the factory instead of instantiating objects directly.
3. Makes adding new types of objects easier without changing client code.

• GUI frameworks where a button factory creates buttons depending on the operating system (Windows, Mac).
• Payment systems where a payment factory creates different payment methods (CreditCard, PayPal, UPI).

// Product interface
interface Button {
    void render();
}
// Concrete Products
class WindowsButton implements Button {
    public void render() { System.out.println("Windows Button"); }
}
class MacButton implements Button {
    public void render() { System.out.println("Mac Button"); }
}
// Factory
class ButtonFactory {
    public static Button createButton(String type) {
        if(type.equals("Windows")) return new WindowsButton();
        else if(type.equals("Mac")) return new MacButton();
        return null;
    }
}
// Client
Button button = ButtonFactory.createButton("Windows");
button.render();

Step 4: Structural Design Patterns

Structural Design Patterns Examples: 

• Adapter – Allows incompatible interfaces to work together by acting as a bridge.
• Bridge – Decouples abstraction from implementation so both can evolve independently.
• Composite – Composes objects into tree structures to represent part-whole hierarchies.
• Decorator – Dynamically adds new responsibilities to objects without altering their structure.
• Facade – Provides a simplified interface to a complex subsystem.
• Flyweight – Reduces memory usage by sharing common parts of objects instead of duplicating them.
• Proxy – Provides a surrogate or placeholder for another object to control access.

• When you want to compose objects into complex structures without tight coupling.
• When new components need to be added without changing existing code.
• When you need flexible and efficient ways to share objects or data.
• When simplifying complex object hierarchies for easier maintenance.

Example 1: Adapter Pattern

1. The adapter class implements the target interface and translates calls to the adaptee class.
2. Client code uses the adapter without knowing about the underlying adaptee.

• Integrating a legacy system with a modern application.
• Connecting different data formats or APIs.

Example Code (Java):

// Target interface
interface MediaPlayer {
    void play(String audioType, String fileName);
}
// Adaptee
class AdvancedMediaPlayer {
    void playVlc(String fileName) { System.out.println("Playing VLC " + fileName); }
}
// Adapter
class MediaAdapter implements MediaPlayer {
    AdvancedMediaPlayer advancedMusicPlayer = new AdvancedMediaPlayer();
    public void play(String audioType, String fileName) {
        if(audioType.equals("vlc")) {
            advancedMusicPlayer.playVlc(fileName);
        }
    }
}
// Client
MediaPlayer player = new MediaAdapter();
player.play("vlc", "song.vlc");

Example 2: Decorator Pattern

1. Wrap the original object with decorator classes that add new behavior.
2. Multiple decorators can be stacked to combine behaviors.

• Adding features to GUI components like scrollbars or borders.
• Enhancing stream functionality in I/O libraries.

// Component
interface Coffee {
    double cost();
}
// Concrete Component
class SimpleCoffee implements Coffee {
    public double cost() { return 5; }
}
// Decorator
class MilkDecorator implements Coffee {
    Coffee coffee;
    MilkDecorator(Coffee coffee) { this.coffee = coffee; }
    public double cost() { return coffee.cost() + 2; }
}
// Client
Coffee coffee = new MilkDecorator(new SimpleCoffee());
System.out.println(coffee.cost()); // Output: 7

Step 5: Behavioral Design Patterns

• Chain of Responsibility – Passes a request along a chain of handlers until one processes it.
• Command – Encapsulates a request as an object, allowing parameterization and queuing of actions.
• Interpreter – Defines a grammar and interprets sentences in a specific language.
• Iterator – Provides a standard way to traverse elements of a collection without exposing its details.
• Mediator – Simplifies communication by centralizing complex object interactions in a mediator.
• Memento – Captures and restores an object’s internal state without violating encapsulation.
• Observer – Establishes a one-to-many dependency where objects update automatically when state changes.

• When you need dynamic assignment of responsibilities between objects.
• When objects must interact in complex ways without tight coupling.
• When implementing algorithms or workflows that can change at runtime.
• When designing event-driven systems or handling notifications.

Example 1: Observer Pattern

• The subject maintains a list of observers.
• Observers register to the subject to receive updates.
• When the subject changes, all observers are notified.

• Implementing event listeners in GUI frameworks.
• Real-time notification systems, e.g., stock price updates.

// Subject
interface Subject {
    void attach(Observer o);
    void detach(Observer o);
    void notifyObservers();
}

// Observer
interface Observer {
    void update(String message);
}
// Concrete Subject
class NewsAgency implements Subject {
    private List observers = new ArrayList<>();
    private String news;
    
    public void attach(Observer o) { observers.add(o); }
    public void detach(Observer o) { observers.remove(o); }
    
    public void notifyObservers() {
        for(Observer o : observers) o.update(news);
    }
    
    public void setNews(String news) {
        this.news = news;
        notifyObservers();
    }
}
// Concrete Observer
class NewsChannel implements Observer {
    private String news;
    public void update(String news) { this.news = news; System.out.println("NewsChannel: " + news); }
}
// Client
NewsAgency agency = new NewsAgency();
NewsChannel channel = new NewsChannel();
agency.attach(channel);
agency.setNews("New Design Pattern Released!");

Example 2: Strategy Pattern

1. Context class uses a strategy interface.
2. Different strategies implement the algorithm differently.
3. Context can switch strategies at runtime without changing client code.

• Payment systems with multiple payment methods (CreditCard, PayPal, UPI).
• Sorting algorithms where the algorithm can be switched dynamically.

// Strategy interface
interface PaymentStrategy {
    void pay(int amount);
}

// Concrete Strategies
class CreditCardPayment implements PaymentStrategy {
    public void pay(int amount) { System.out.println("Paid " + amount + " using Credit Card."); }
}
class PayPalPayment implements PaymentStrategy {
    public void pay(int amount) { System.out.println("Paid " + amount + " using PayPal."); }
}
// Context
class ShoppingCart {
    private PaymentStrategy paymentStrategy;
    public void setPaymentStrategy(PaymentStrategy paymentStrategy) { this.paymentStrategy = paymentStrategy; }
    public void checkout(int amount) { paymentStrategy.pay(amount); }
}
// Client
ShoppingCart cart = new ShoppingCart();
cart.setPaymentStrategy(new PayPalPayment());
cart.checkout(500)

Step 6: Advanced Concepts

Design patterns form the foundation of clean and maintainable software design. To become proficient, it’s important to understand advanced concepts that complement and extend design patterns.

1. SOLID Principles

1. S – Single Responsibility Principle (SRP): A class should have only one reason to change. Ensures focused and maintainable code.
2. O – Open/Closed Principle (OCP): Software entities should be open for extension, closed for modification. Supports adding new functionality without changing existing code.
3. L – Liskov Substitution Principle (LSP): Subtypes must be replaceable for their base types without altering correctness. Ensures reliable polymorphism in class hierarchies
4. I – Interface Segregation Principle (ISP): Clients should not be forced to depend on interfaces they don’t use. Promotes smaller, focused interfaces rather than large general ones.
5. D – Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules; both should depend on abstractions. Reduces tight coupling and improves code flexibility.

2. Architectural Patterns

• Layered Architecture (n-tier): It separates system into presentation, business, and data layers, also improves modularity and maintainability.
• Client-Server Architecture: System is divided into clients (requesters) and servers (providers) which ensures centralized control and resource sharing.
• Microservices Architecture: Microservices System is composed of independent, loosely coupled services. It promotes scalability, deployment flexibility, and resilience.
• Event-Driven Architecture: Components react to events instead of synchronous calls. Ideal for highly dynamic and responsive systems.
• MVC (Model-View-Controller): Separates data (Model), UI (View), and logic (Controller). It enhances maintainability and testability of UI applications.

3. Advanced Design Techniques

Step 7: Tools for Design Patterns

1. Implementation Tools

1. IDE Support (Eclipse, IntelliJ, VS Code, NetBeans): 

• They provide code templates, support for refactoring, and pattern snippets.
• Example: IntelliJ includes built-in inspections that detect anti-patterns.

• These tools help visualize class diagrams, sequence diagrams, and object interactions.
• They are essential for understanding pattern relationships.

• Spring (Java) implements Dependency Injection and Singleton.
• .NET Core offers built-in support for Factory and Observer.
• React and Angular implement Observer (for state management) and MVC/MVVM.

4. Testing Tools (JUnit, NUnit, Mocha, Jest)

• These tools ensure that the pattern implementation works correctly.
• They are especially useful for Strategy, Observer, and Template Method patterns.

2. Management Tools

• Version Control Systems (Git, GitHub, GitLab, Bitbucket): They manage pattern evolution and support refactoring.
• Project Management Tools (Jira, Trello, Asana): These tools track design decisions and monitor the progress of pattern adoption.
• Documentation Tools (Confluence, Notion, Markdown, PlantUML): They maintain pattern guidelines, UML diagrams, and architecture documentation.
• Collaboration Tools (Slack, Microsoft Teams, Zoom, Miro): These tools enable team discussions, brainstorming, and pattern design workshops.

Step 8: Design Pattern Best Practices

• Don’t force a design pattern.
• Analyze the problem and apply the right pattern for the situation.

2. Favor Composition over Inheritance

• Use object composition (like Strategy and Decorator) instead of deep inheritance trees.
• This improves flexibility and reusability.

• Avoid over-engineering with patterns.
• Start with simple solutions and introduce patterns only when complexity requires it.

Many real-world systems use combinations of patterns.

• Example: Factory combined with Singleton (for a single factory instance).
• Example: Strategy combined with Template Method (for flexible workflows).

• Always record the reasons for choosing a particular pattern.
• This helps future developers understand the context.

• Patterns are not permanent; they change with requirements.
• Regular refactoring ensures the code remains maintainable and scalable.

• God Object (violates SRP).
• Singleton Abuse (using everywhere leads to tight coupling).
• Over-engineering (adding unnecessary abstraction).

Conclusion

Design patterns are not just theoretical concepts. They are tried-and-true solutions that connect software design with practical use. By knowing the categories of patterns (Creational, Structural, Behavioral), applying SOLID principles, using the right tools, and following best practices, developers can create systems that are scalable, reusable, and easy to maintain.

Enroll in our Java Solution Architect Certification Training & ASP.NET Core Certification Training to gain hands-on experience and apply these patterns with ease.

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