Strategy

-- defines a family of algorithms , encapsualtes each one, and makes them interchangeable. It letss the algorithm vary independently from the client.

Observer

-- defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updates automatically.

Decorator

-- attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Factory

-- defines an interface for creating an object, but lets subclasses decide which class to instantiate.

Abstract Factory

-- provides an interface for creating families of related or dependent objects without specifying their concrete classes.

Singleton

-- ensures that a class has only one instance, and provides a global point of access to it.

Command

-- encapsulates a request as an object, thereby letting you parameterize other objects with different requests, and support undoable operations.

Adapter

-- converts the interface of a class into another interface the client expects.

Template

-- defines the skeleton of an algorithm in a method, deferring some steps to subclasses.

Iterator

-- provides a way to access the elemtns of an aggregate object sequentially without exposing its underlying representation.

Composite

-- allows you to compose objects into tree structures to represent part-whole hierarchies, and to treat these compositions uniformly.

State

-- allows an object to alter its behavior when its internal state changes.

Proxy

-- provides a surrogate or placeholder for another object to control access to it.