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.
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 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 method
Defer the exact steps of an algorithm to a subclass
Visitor
Defines a new operation to a class without change
