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.
Creational design patterns
This design patterns is 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
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
This design patterns is 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
This design patterns is 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 method
Defer the exact steps of an algorithm to a subclass
Visitor
Defines a new operation to a class without change
Abstract Factory Design Pattern
Intent
• Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
• A hierarchy that encapsulates: many possible “platforms”, and the construction of a suite of “products”.
• The new operator considered harmful.
Example
The purpose of the Abstract Factory is to provide an interface for creating families of related objects, without specifying concrete classes. This pattern is found in the sheet metal stamping equipment used in the manufacture of Japanese automobiles. The stamping equipment is an Abstract Factory which creates auto body parts. The same machinery is used to stamp right hand doors, left hand doors, right front fenders, left front fenders, hoods, etc. for different models of cars. Through the use of rollers to change the stamping dies, the concrete classes produced by the machinery can be changed within three minutes.
Factory Method Design Pattern
Intent
• Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
• Defining a “virtual” constructor.
• The new operator considered harmful.
Example
The Factory Method defines an interface for creating objects, but lets subclasses decide which classes to instantiate. Injection molding presses demonstrate this pattern. Manufacturers of plastic toys process plastic molding powder, and inject the plastic into molds of the desired shapes. The class of toy (car, action figure, etc.) is determined by the mold.
Object Pool Design Pattern
Intent
Object pooling can offer a significant performance boost; it is most effective in situations where the cost of initializing a class instance is high, the rate of instantiation of a class is high, and the number of instantiations in use at any one time is low.
Do you like bowling? If you do, you probably know that you should change your shoes when you getting the bowling club. Shoe shelf is wonderful example of Object Pool. Once you want to play, you’ll get your pair (aquireReusable) from it. After the game, you’ll return shoes back to the shelf (releaseReusable).
Prototype Design Pattern
Intent
• Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
• Co-opt one instance of a class for use as a breeder of all future instances.
• The new operator considered harmful.
Example
The Prototype pattern specifies the kind of objects to create using a prototypical instance. Prototypes of new products are often built prior to full production, but in this example, the prototype is passive and does not participate in copying itself. The mitotic division of a cell - resulting in two identical cells - is an example of a prototype that plays an active role in copying itself and thus, demonstrates the Prototype pattern. When a cell splits, two cells of identical genotvpe result. In other words, the cell clones itself.
Singleton Design Pattern
Intent
• Ensure a class has only one instance, and provide a global point of access to it.
• Encapsulated “just-in-time initialization” or “initialization on first use”.
Example
The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. It is named after the singleton set, which is defined to be a set containing one element. The office of the President of the United States is a Singleton. The United States Constitution specifies the means by which a president is elected, limits the term of office, and defines the order of succession. As a result, there can be at most one active president at any given time. Regardless of the personal identity of the active president, the title, “The President of the United States” is a global point of access that identifies the person in the office.
Adapter Design Pattern
Intent
• Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.
• Wrap an existing class with a new interface.
• Impedance match an old component to a new system
Example
The Adapter pattern allows otherwise incompatible classes to work together by converting the interface of one class into an interface expected by the clients. Socket wrenches provide an example of the Adapter. A socket attaches to a ratchet, provided that the size of the drive is the same. Typical drive sizes in the United States are 1/2” and 1/4”. Obviously, a 1/2” drive ratchet will not fit into a 1/4” drive socket unless an adapter is used. A 1/2” to 1/4” adapter has a 1/2” female connection to fit on the 1/2” drive ratchet, and a 1/4” male connection to fit in the 1/4” drive socket
Decorator Design Pattern
Intent
• Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
• Client-specified embellishment of a core object by recursively wrapping it.
• Wrapping a gift, putting it in a box, and wrapping the box.
Example
The Decorator attaches additional responsibilities to an object dynamically. The ornaments that are added to pine or fir trees are examples of Decorators. Lights, garland, candy canes, glass ornaments, etc., can be added to a tree to give it a festive look. The ornaments do not change the tree itself which is recognizable as a Christmas tree regardless of particular ornaments used. As an example of additional functionality, the addition of lights allows one to “light up” a Christmas tree.
Although paintings can be hung on a wall with or without frames, frames are often added, and it is the frame which is actually hung on the wall. Prior to hanging, the paintings may be matted and framed, with the painting, matting, and frame forming a single visual component.
Facade Design Pattern
Intent
• Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
• Wrap a complicated subsystem with a simpler interface.
Example
The Facade defines a unified, higher level interface to a subsystem that makes it easier to use. Consumers encounter a Facade when ordering from a catalog. The consumer calls one number and speaks with a customer service representative. The customer service representative acts as a Facade, providing an interface to the order fulfillment department, the billing department, and the shipping department.
Reference:
http://sourcemaking.com/design_patterns
