Tell me about design patterns: groups and examples.

Design patterns are proven solutions to common software development problems. They can be divided into three main categories:

1. Creational — responsible for object creation, hiding complexity and providing flexibility:

   • Singleton — ensures a class has only one instance.
   • Factory Method — defines an interface for creating an object, allowing subclasses to alter the type of objects that will be created.
   • Abstract Factory — provides an interface for creating families of related or dependent objects.
   • Builder — separates the construction of a complex object from its representation.
   • Prototype — creates new objects by copying existing ones.

2. Structural — define ways to compose classes and objects:

   • Adapter — allows objects with incompatible interfaces to work together.
   • Decorator — dynamically adds responsibilities to objects.
   • Facade — provides a simplified interface to a complex subsystem.
   • Composite — allows grouping objects into tree structures.
   • Proxy — provides a surrogate or placeholder for another object to control access.

3. Behavioral — responsible for interaction between objects:

   • Observer — notifies subscribers about changes in an object's state.
   • Strategy — defines a family of algorithms, encapsulates each one, and makes them interchangeable.
   • Command — encapsulates a request as an object, allowing parameterization of clients.
   • Iterator — provides a way to access elements of a collection sequentially.
   • State — allows an object to alter its behavior when its internal state changes.

Understanding and applying patterns helps create flexible, extendable, and maintainable architectures.