Design Patterns

In software engineering, a design pattern is a general repeatable solution to a commonly occurring problem in software design. A design pattern isn't a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that can be used in many different situations.

Uses of Design Patterns

Design patterns can speed up the development process by providing tested, proven development paradigms. Effective software design requires considering issues that may not become visible until later in the implementation. Reusing design patterns helps to prevent subtle issues that can cause major problems and improves code readability for coders and architects familiar with the patterns.

Often, people only understand how to apply certain software design techniques to certain problems. These techniques are difficult to apply to a broader range of problems. Design patterns provide general solutions, documented in a format that doesn't require specifics tied to a particular problem.

In addition, patterns allow developers to communicate using well-known, well understood names for software interactions. Common design patterns can be improved over time, making them more robust than ad-hoc designs.

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 effectively to get the job done.

Abstract Factory

Creates an instance of several families of classes

Creates an instance of several families of classes Builder

Separates object construction from its representation

Separates object construction from its representation Factory Method

Creates an instance of several derived classes

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

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

A fully initialized instance to be copied or cloned Singleton

A class of which only a single instance can exist

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

Match interfaces of different classes Bridge

Separates an object’s interface from its implementation

Separates an object’s interface from its implementation Composite

A tree structure of simple and composite objects

A tree structure of simple and composite objects Decorator

Add responsibilities to objects dynamically

Add responsibilities to objects dynamically Facade

A single class that represents an entire subsystem

A single class that represents an entire subsystem Flyweight

A fine-grained instance used for efficient sharing

A fine-grained instance used for efficient sharing Private Class Data

Restricts accessor/mutator access

Private Class Data Restricts accessor/mutator access Proxy

An object representing another object

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

A way of passing a request between a chain of objects Command

Encapsulate a command request as an object

Encapsulate a command request as an object Interpreter

A way to include language elements in a program

A way to include language elements in a program Iterator

Sequentially access the elements of a collection

Sequentially access the elements of a collection Mediator

Defines simplified communication between classes

Defines simplified communication between classes Memento

Capture and restore an object's internal state

Capture and restore an object's internal state Null Object

Designed to act as a default value of an object

Designed to act as a default value of an object Observer

A way of notifying change to a number of classes

A way of notifying change to a number of classes State

Alter an object's behavior when its state changes

State Alter an object's behavior when its state changes Strategy

Encapsulates an algorithm inside a class

Encapsulates an algorithm inside a class Template method

Defer the exact steps of an algorithm to a subclass

Defer the exact steps of an algorithm to a subclass Visitor

Defines a new operation to a class without change

Criticism