What are the top must-know software design patterns?

The top must-know software design patterns are Singleton, Factory Method, Abstract Factory, Builder, Prototype, Adapter, Composite, Proxy, Facade, Flyweight, Observer, Strategy, Command, State, and Mediator. Every one of them falls into one of three types: creational, structural, or behavioral.

This guide explains what each of these design patterns does, when to use it, and shows a short code example for every pattern.

What Are Software Design Patterns?

A software design pattern is a reusable, named solution to a design problem that shows up again and again in object-oriented code. A pattern is not a library or a piece of code you copy. It is a template: it describes the problem, the solution, and the trade-offs, and you adapt it to your own codebase.

The idea was popularized by the "Gang of Four" (GoF) book, Design Patterns: Elements of Reusable Object-Oriented Software, which cataloged 23 classic patterns. You do not need all 23 to write good software or to pass an interview. The 15 patterns in this guide cover the vast majority of real-world use.

The 3 Types of Design Patterns

Design patterns are grouped into three categories: creational, structural, and behavioral. Each category solves a different kind of problem.

Why Design Patterns Matter

• They solve proven problems: Patterns are battle-tested solutions, so you avoid reinventing (and re-breaking) the wheel.
• They give teams a shared vocabulary: Saying "use a Facade here" communicates a full design in three words.
• They improve code quality: Patterns push you toward loosely coupled, reusable, and maintainable structures.
• They prepare you for interviews: Object-oriented design interviews and low-level design (LLD) rounds lean heavily on these 15 patterns.

Creational Design Patterns

Creational design patterns deal with object creation. They hide the construction logic so the rest of the system does not depend on how its objects are built. Here are the five creational patterns worth knowing cold.

1. Singleton Pattern

• What it does: Ensures that a class has only one instance and provides a global point of access to it.
• Example: A database connection pool. Creating multiple pools would flood the database with connections. The Singleton pattern guarantees one pool, accessible everywhere.

  public class DatabaseConnection {
      private static DatabaseConnection instance;
  
      private DatabaseConnection() {}
  
      public static synchronized DatabaseConnection getInstance() {
          if (instance == null) {
              instance = new DatabaseConnection();
          }
          return instance;
      }
  }

2. Factory Method Pattern

• What it does: Defines an interface for creating an object, but lets subclasses decide which concrete class to instantiate.
• Example: A logistics system where the transport type is decided at runtime. Each subclass picks its own transport without changing the delivery-planning code.

  abstract class Logistics {
      public abstract Transport createTransport();
  
      public void planDelivery() {
          Transport transport = createTransport();
          transport.deliver();
      }
  }
  class RoadLogistics extends Logistics {
      @Override
      public Transport createTransport() {
          return new Truck();
      }
  }

3. Abstract Factory Pattern

• What it does: Provides an interface for creating families of related objects without specifying their concrete classes.
• Example: A UI toolkit where buttons and checkboxes must look consistent per platform. Each factory produces a matching family of components.

  interface GUIFactory {
      Button createButton();
      Checkbox createCheckbox();
  }
  class WinFactory implements GUIFactory {
      public Button createButton() { return new WinButton(); }
      public Checkbox createCheckbox() { return new WinCheckbox(); }
  }

4. Builder Pattern

• What it does: Constructs complex objects step by step, so the same construction code can produce different representations.
• Example: A meal planner where a meal is assembled from burgers, drinks, and sides. The builder absorbs all the variations.

  class MealBuilder {
      private Meal meal = new Meal();
  
      public MealBuilder addBurger(String type) { /* ... */ }
      public MealBuilder addDrink(String type) { /* ... */ }
      public Meal build() { return meal; }
  }

5. Prototype Pattern

• What it does: Creates new objects by cloning an existing object (the prototype). Useful when building an object from scratch is more expensive than copying one.
• Example: A game that needs thousands of similar tree objects. Instead of constructing each tree, clone a prototype and tweak its properties.

  class Tree implements Cloneable {
      private String type;
      private Color color;
      private Position position;
  
      public Tree clone() {
          // Create a deep copy of this tree object.
      }
  }

Creational patterns make a system independent of how its objects are created, composed, and represented. That independence is what keeps construction changes from rippling through the codebase.

Structural Design Patterns

Structural design patterns are concerned with how classes and objects are composed into larger structures. They simplify a design by identifying clean relationships between entities. These are the five structural patterns to know.

1. Adapter Pattern (Wrapper)

• What it does: Lets an existing class work through a different interface, without modifying its source code.
• Example: Your application produces customer data in JSON, but a third-party billing system expects XML. An adapter converts between the two so neither side changes.

  // Existing interface (XML input)
  interface BillingSystem {
      void processBilling(String customerDataXml);
  }
  
  // Adapter
  class BillingAdapter implements BillingSystem {
      private JsonBillingSystem jsonBillingSystem = new JsonBillingSystem();
  
      @Override
      public void processBilling(String customerDataJson) {
          String customerDataXml = convertJsonToXml(customerDataJson);
          jsonBillingSystem.processBilling(customerDataXml);
      }
  
      private String convertJsonToXml(String json) {
          // Conversion logic
          return "<xml>";
      }
  }

2. Composite Pattern

• What it does: Composes objects into tree structures to represent part-whole hierarchies, so clients treat single objects and groups of objects the same way.
• Example: A graphics editor where shapes can be grouped into diagrams. Moving or drawing works identically on one circle or a whole group.

  interface Graphic {
      void move(int x, int y);
      void draw();
  }
  
  class CompositeGraphic implements Graphic {
      private List<Graphic> childGraphics = new ArrayList<>();
  
      public void add(Graphic graphic) {
          childGraphics.add(graphic);
      }
  
      @Override
      public void move(int x, int y) {
          for (Graphic graphic : childGraphics) {
              graphic.move(x, y);
          }
      }
  
      @Override
      public void draw() {
          for (Graphic graphic : childGraphics) {
              graphic.draw();
          }
      }
  }

3. Proxy Pattern

• What it does: Provides a placeholder for another object to control access to it: for security, caching, or lazy loading of an expensive object.
• Example: An image loader proxy that defers loading a high-resolution image from disk until the image actually needs to be displayed.

  interface Image {
      void display();
  }
  
  class HighResolutionImage implements Image {
      public HighResolutionImage(String imagePath) {
          // Load image from disk (heavy operation)
      }
  
      @Override
      public void display() {
          // Display the image
      }
  }
  
  class ImageLoaderProxy implements Image {
      private String imagePath;
      private HighResolutionImage highResImage;
  
      public ImageLoaderProxy(String imagePath) {
          this.imagePath = imagePath;
      }
  
      @Override
      public void display() {
          if (highResImage == null) {
              highResImage = new HighResolutionImage(imagePath);
          }
          highResImage.display();
      }
  }

4. Facade Pattern

• What it does: Provides one simplified interface to a complex subsystem of classes, a library, or a framework.
• Example: A home theater facade that hides the projector, sound system, and lights behind a single watchMovie() call.

  class HomeTheaterFacade {
      private Projector projector;
      private SoundSystem soundSystem;
      private Lights lights;
  
      public HomeTheaterFacade(Projector projector, SoundSystem soundSystem, Lights lights) {
          this.projector = projector;
          this.soundSystem = soundSystem;
          this.lights = lights;
      }
  
      public void watchMovie() {
          lights.dim();
          soundSystem.turnOn();
          projector.turnOn();
          // Setup more components...
      }
  }

5. Flyweight Pattern

• What it does: Cuts the cost of creating huge numbers of similar objects by sharing their common state instead of duplicating it in every object.
• Example: A text editor rendering millions of characters. Each character object shares the intrinsic state (the glyph), while position and font are passed in from outside.

  class Character {
      private char glyph; // Intrinsic state
      public Character(char glyph) {
          this.glyph = glyph;
      }
      public void display(int fontSize, int positionX, int positionY) {
          // Display character with extrinsic state
      }
  }

Structural patterns keep an architecture easy to understand and extend. In interviews, Adapter, Facade, and Proxy come up constantly because real systems integrate with other systems all the time.