Design patterns are solutions to software design problems you find again and again in real-world application development. Patterns are about reusable designs and interactions of objects. They are categorized in three groups: Creational, Structural, and Behavioral.
The Singleton design pattern ensures a class has only one instance and provide a global point of access to it.
The Factory Method design pattern defines an interface for creating an object, but let subclasses decide which class to instantiate. This pattern lets a class defer instantiation to subclasses.
The Decorator design pattern attaches additional responsibilities to an object dynamically. This pattern provide a flexible alternative to subclassing for extending functionality.
The Chain of Responsibility design pattern avoids coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. This pattern chains the receiving objects and passes the request along the chain until an object handles it.
