What are the main design patterns?

Design patterns are proven solutions to common problems encountered in software design. They provide a standardized approach to structuring code, enhancing readability, maintainability, and scalability. The main design patterns are typically categorized into three groups: Creational, Structural, and Behavioral. Below is an overview of the most prominent design patterns within each category:

Creational Design Patterns

These patterns deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. They abstract the instantiation process, making a system independent of how its objects are created, composed, and represented.

1. Singleton

   • Purpose: Ensures a class has only one instance and provides a global point of access to it.
   • Use Case: Managing a single resource, such as a configuration manager or a connection pool.

2. Factory Method

   • Purpose: Defines an interface for creating an object but lets subclasses alter the type of objects that will be created.
   • Use Case: When a class cannot anticipate the class of objects it needs to create.

3. Abstract Factory

   • Purpose: Provides an interface for creating families of related or dependent objects without specifying their concrete classes.
   • Use Case: When the system needs to be independent of how its products are created, composed, and represented.

4. Builder

   • Purpose: Separates the construction of a complex object from its representation, allowing the same construction process to create different representations.
   • Use Case: When creating complex objects with numerous optional parameters.

5. Prototype

   • Purpose: Specifies the kinds of objects to create using a prototypical instance and creates new objects by copying this prototype.
   • Use Case: When the cost of creating a new object is expensive or complex.

Structural Design Patterns

These patterns concern class and object composition. They help ensure that if one part of a system changes, the entire system doesn’t need to do the same.

1. Adapter

   • Purpose: Allows incompatible interfaces to work together by converting the interface of a class into another interface clients expect.
   • Use Case: Integrating legacy systems with new systems.

2. Bridge

   • Purpose: Decouples an abstraction from its implementation so that the two can vary independently.
   • Use Case: When both the class and what it does vary often.

3. Composite

   • Purpose: Composes objects into tree structures to represent part-whole hierarchies, allowing clients to treat individual objects and compositions uniformly.
   • Use Case: Implementing hierarchical structures like file systems.

4. Decorator

   • Purpose: Adds additional responsibilities to an object dynamically without altering its structure.
   • Use Case: Enhancing the functionality of individual objects.

5. Facade

   • Purpose: Provides a simplified interface to a complex subsystem, making it easier to use.
   • Use Case: Simplifying interactions with a library or framework.

6. Flyweight

   • Purpose: Reduces the cost of creating and manipulating a large number of similar objects by sharing as much data as possible.
   • Use Case: Managing memory usage in applications that require numerous objects, such as text editors.

7. Proxy

   • Purpose: Provides a surrogate or placeholder for another object to control access to it.
   • Use Case: Implementing lazy initialization, access control, or logging.

Behavioral Design Patterns

These patterns are concerned with algorithms and the assignment of responsibilities between objects. They help manage complex control flows and communication between objects.

1. Observer

   • Purpose: Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
   • Use Case: Implementing event handling systems, such as GUI frameworks.

2. Strategy

   • Purpose: Defines a family of algorithms, encapsulates each one, and makes them interchangeable, allowing the algorithm to vary independently from clients that use it.
   • Use Case: Implementing different sorting strategies or payment methods.

3. Command

   • Purpose: Encapsulates a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations.
   • Use Case: Implementing undoable operations in applications.

4. Iterator

   • Purpose: Provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
   • Use Case: Traversing collections like lists, trees, or graphs.

5. Mediator

   • Purpose: Defines an object that encapsulates how a set of objects interact, promoting loose coupling by keeping objects from referring to each other explicitly.
   • Use Case: Managing complex communications in GUI applications.

6. State

   • Purpose: Allows an object to alter its behavior when its internal state changes, making the object appear to change its class.
   • Use Case: Implementing state machines, such as game character states.

7. Template Method

   • Purpose: Defines the skeleton of an algorithm in an operation, deferring some steps to subclasses, allowing them to redefine certain steps without changing the algorithm’s structure.
   • Use Case: Implementing invariant parts of an algorithm while allowing flexibility in specific steps.

8. Chain of Responsibility

   • Purpose: Passes a request along a chain of handlers, allowing multiple objects to handle the request without specifying the handler explicitly.
   • Use Case: Implementing logging systems or event handling mechanisms.

9. Visitor

   • Purpose: Represents an operation to be performed on the elements of an object structure, allowing new operations to be added without modifying the classes of the elements.
   • Use Case: Performing operations like serialization, validation, or formatting on complex object structures.

10. Memento

    • Purpose: Captures and externalizes an object’s internal state without violating encapsulation, allowing the object to be restored to this state later.
    • Use Case: Implementing undo mechanisms in applications.

Additional Design Patterns

Beyond the classic Gang of Four patterns, several other design patterns are widely used:

• MVC (Model-View-Controller)

  • Purpose: Separates an application into three interconnected components to separate internal representations from user interfaces.
  • Use Case: Web applications and GUI-based applications.

• MVVM (Model-View-ViewModel)

  • Purpose: Similar to MVC but adds a ViewModel to handle data binding between the Model and the View.
  • Use Case: WPF applications, frontend frameworks like Angular.

• CQRS (Command Query Responsibility Segregation)

  • Purpose: Separates read and write operations into different models, optimizing each for its specific purpose.
  • Use Case: Systems with high read and write loads, like e-commerce platforms.

Conclusion

Understanding and applying design patterns effectively can significantly improve the quality and maintainability of your software designs. Familiarizing yourself with these patterns, knowing when to apply them, and understanding their benefits and trade-offs are essential skills for any software developer or architect.

For further reading and practical examples, consider exploring resources like Design Patterns: Elements of Reusable Object-Oriented Software by the Gang of Four and online platforms such as Refactoring.Guru which provide detailed explanations and code examples.