Design Patterns

General, reusable solution to a commonly occurring problem within a given context in software design.

Design patterns are being used in object oriented programming.
Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved.

Design Patterns can be classified in 3 categories: Creational, Structural and Behavioral patterns.

Creational Design Patterns

These design patterns are all about class instantiation.

This pattern can be further divided into class-creation patterns and object-creational patterns.
While class-creation patterns use inheritance effectively in the instantiation process, object-creation patterns use delegation (create child object using parent object) effectively to get the job done.

• Abstract Factory - Creates an instance of several families of classes.
• Builder - Separates object construction from its representation.
• Factory Method - Creates an instance of several derived classes.
• Object Pool - Avoid expensive acquisition and release of resources by recycling objects that are no longer in use.
• Prototype - A fully initialized instance to be copied or cloned.
• Singleton - A class of which only a single instance can exist.

Structural  Design Patterns

These design patterns are all about Class and Object composition.

Structural class-creation patterns use inheritance to compose interfaces.
Structural object-patterns define ways to compose objects to obtain new functionality.

• Adapter - Match interfaces of different classes
• Bridge - Separates an object’s interface from its implementation
• Composite - A tree structure of simple and composite objects
• Decorator - Add responsibilities to objects dynamically
• Facade - A single class that represents an entire subsystem
• Flyweight - A fine-grained instance used for efficient sharing
• Private Class Data - Restricts accessor/mutator access
• Proxy - An object representing another object

Behavioral Design Patterns

These design patterns are all about Class's objects communication.
Behavioral patterns are those patterns that are most specifically concerned with communication between objects.

• Chain of responsibility - A way of passing a request between a chain of objects
• Command - Encapsulate a command request as an object
• Interpreter - A way to include language elements in a program
• Iterator - Sequentially access the elements of a collection
• Mediator - Defines simplified communication between classes
• Memento - Capture and restore an object's internal state
• Null Object - Designed to act as a default value of an object
• Observer - A way of notifying change to a number of classes
• State - Alter an object's behavior when its state changes
• Strategy - Encapsulates an algorithm inside a class
• Template - Defer the exact steps of an algorithm to a subclass
• Visitor - Defines a new operation to a class without change

Best Design Patterns

The best design patterns are:

• Singleton
• Factory Method
• Strategy
• Observer
• Builder
• Adapter
• State