Allows behavior to be dynamically added to individual objects without modifying their code, promoting flexibility and adherence to the open/closed principle.
Provides an interface for creating objects in a superclass, but allows subclasses to alter the type of objects that will be created, promoting flexibility and reusability in object creation.
Defines a one-to-many dependency between objects, so when one object changes state, all its dependents are notified and updated automatically.
Encapsulates the logic for accessing and managing data sources, providing a centralized interface for data operations while promoting separation of concerns and improving testability.
Ensures a class has only one instance, providing a single point of access for shared resources like logging, configuration, or database connections.