Design Patterns

The 24 design patterns are broken down into three groups (Gamma Categories after Erich Gamma one of the gang of Four), in a word, we can say Creational is about instantiation, and Structural is the blueprint, and Behavioral is the pattern of the relationship among objects:

Creational	are used to create objects for a suitable class that serves as a solution for a problem. Creational patterns allow objects to be created in a system without having to identify a specific class type in the code which cuts down on code. Creational design patterns are composed of two dominant ideas. One is encapsulating knowledge about which concrete classes the system uses. Another is hiding how instances of these concrete classes are created and combined.
Structural	are concerned with how classes and objects are composed to form large structures, they use inheritance to compose interfaces or implementations. They help you guarantee that when one of the parts changes, the entire structure does not need to change.
Behavioral	describe interactions between objects and focus on how objects communicate with each other, they are concerned with algorithms and the assignment of responsibilities between objects.

The below table details each pattern grouped by either Creational, Structural and Behavioral, I have also tried to add where you might use an appropriate design pattern. Don't worry if some of this is a bit cryptic I will demostrate via code each pattern. Use this table as a quick reference guide for the purpose and where to use the appropriate pattern/principle.

Creational
Builder	Purpose	useful for dynamic creation of objects
	Uses	building strings, build a person object, DOM/XML creation, streams(), etc (all about chaining member fields to build the object)
	Notes	use 'return this' in your code so that you can use chaining
Factories	Purpose	Factory - Creates objects without exposing the instantiation logic to the client. Factory Method - Define an interface for creating an object, but let the subclasses decide which class to instantiate.The Factory method lets a class defer instantiation to subclasses Abstract Factory - provides an interface for creating families of related or dependent objects without specifying their concrete classes.
	Uses	Factory - Client just need a class and does not care about which concrete implementation it is getting. Factory Method - Client doesn't know what concrete classes it will be required to create at runtime, but just wants to get a class that will do the job. Abstract Factory - When your system has to create multiple families of products or you want to provide a library ofproducts without exposing the implementation details.
	Notes	Factory Method is used to create one product only but Abstract Factory is about creating families of related or dependent products. Abstract Factory classes are often implemented with Factory Method. Factory Methods are usually called within Template Methods.
Prototype	Purpose	Create objects based upon a template of an existing objects through cloning
	Uses	When you have to instantiate many objects (performance) its better to clone existing and change it
	Notes
Singleton	Purpose	Only one instance of an Object can exist
	Uses	used for logging, drivers objects, caching and thread pool
	Notes	- Private constructor to restrict instantiation of the class from other classes. - Private static variable of the same class that is the only instance of the class. - Public static method that returns the instance of the class, this is the global access point for outer world to get the instance of the singleton class. There are a number of ways a singleton can be implemented 1. Eager initialization 2. Static block initialization 3. Lazy Initialization 4. Thread Safe Singleton 5. Bill Pugh Singleton Implementation (inner static help class) 6. Using Reflection to destroy Singleton Pattern 7. Enum Singleton (cannot use lazy initialization but overcomes reflection issues) 8. Serialization and Singleton (deserialize issues (creates another instance of the singleton object)
Structural
Adapter	Purpose	Permit classes with disparate interfaces to work together by creating a common object by which they may communicate and interact, no new functionality is created.
	Uses	convert one thing into another using an adapter in the middle, convert company tax to HRMC tax
	Notes
Decorator	Purpose	Allows for the dynamic wrapping of objects in order to modify their existing responsibilities and behaviors, cuts down on the number of different permutations.
	Uses	cup of milky (object), extra shot (object), hazel nutty (object), frothy milk (object) - all objects are wrapped one inside the other
	Notes	- Bridge you decouple abstraction from implementation by wrapping the object implemented with a class that doesn't necessarily share the same interface of the object. - Decorator modify existing behaviour by wrapping the object implemented with a class that inherits the same interface as the object implemented, so it has the same behaviours.
Facade	Purpose	Supplies a single interface to a set of interfaces within a system
	Uses	use when you have a complex system to simplify things, for example to watch a movie it will make popcorn, dim lights, pull curtains, turn on sound system, put movie on.
	Notes
Flyweight	Purpose	The reuse of many fine grained objects, making the utilization of large numbers of objects more efficient, try to reduce memory footprint by reuse.
	Uses	Many similar objects are used and the storage cost is high (use caching, use arrays to hold different data in an object but use the same object)
	Notes
Proxy	Purpose	Allow access control by acting as a pass through entity or a placeholder object.
	Uses	Security access control, remote invocation service (RMI)
	Notes	There are a number of different proxy types - A remote proxy is responsible for encoding a request and its arguments and for sending (and retrieving) the request (and the response) to the real object. - A virtual proxy may cache additional information about the real subject so that it can postpone the access to it. - A protection proxy checks whether the caller has sufficient access permissions for perform a request. - A Property proxy ?? - A Dynamic proxy used for logging for example
Behavioral
Chain Of Responsibility	Purpose	Gives more than one object an opportunity to handle a request by linking receiving objects together (chain), any object can break he chain.
	Uses	Exception handling, filtering, security authentication (check users roles, credentials, access, etc)
	Notes
Command	Purpose	Use an Object that represents an operation (encapsulates action/s - execute method) that can be rolled back (undo), can also be used for callbacks
	Uses	Recording of history and can be used in call backs, thread queues/pools - decouples thread from the command object action, undoing actions, recording history, GUI buttons
	Notes
Interpreter	Purpose	Textual input (String) needs to be processed (turned into OOP structures), turn grammar into actions
	Uses	Interpreters (Perl, Ruby, Bash Shell), turn text formula into actual calculation formula (Matlab for example)
	Notes
Observer	Purpose	Lets one or more objects be notified of state change in other objects within the system
	Uses	Magazine publishing, Facebook notifications, Software updates (any thing with publish and subscribe)
	Notes
State	Purpose	Changes in state can be explicit or in response to event (Observer Pattern), Depending on your state machine you go from one state to another
	Uses	Gumball Machine, Jukebox, Traffic lights, Kettle (on/off), Record Player, etc
	Notes
Strategy	Purpose	System behavior partially specified at RUNTIME, Many algorithms can be decomposed into higher and lower level parts the high level parts can then be reused
	Uses	Gamer figure could walk, run, swim (we don't know until he/she does it), sorting (bubble sort, brick sort, etc), output text (could be XML, JSON, HTML)
	Notes
Template Method	Purpose	Identifies the framework of an algorithm, allowing implementing classes to define the actual behavior, the class is normally abstract
	Uses	Spring data access, game structure template, computer base template (add cpu, add memory, etc)
	Notes	Can be confused with the strategy class, Strategy pattern defines a family of algorithms and makes them interchangeable. Client code can use different algorithms since the algorithms are encapsulated. Template method defines the outline of an algorithm and lets subclasses part of the algorithm's implementation. So you can have different implementations of an algorithms steps but retain the algorithm's structure

SOLID Princples

Solid principles are coding standards that all developers should know about as they are concepts for developing software in a preper way to avoid a bad design, so the code is more maintainable, understandable and flexible. The 5 concepts are the following:

• Single Responsibility Principle (SRP)
• Open-Closed Principle (OCP)
• Liskov Subsitution Principle (LSP)
• Interface Segregation Principle (ISP)
• Dependency Inversion Principle (DIP)

I have covered the Solid Principles in another section

DRY and KISS Princples

The DRY and KISS principles cover all walks of life and fit well with programming as well

• DRY - Don't Repeat Yourself
• KISS - Keep It Simple Stupid

The DRY principle is stated as 'Every piece of knowledge or logic must have a single, unambiguous representation within a system'. To avoid violating the DRY principle, divide your system into pieces. Divide your code and logic into smaller reusable units and use that code by calling it where you want. Don't write lengthy methods, but divide logic and try to use the existing piece in your method. Less code is good as it saves time and effort, is easy to maintain, and also reduces the chances of bugs.

The KISS principle is descriptive to keep the code simple and clear, making it easy to understand. To avoid violating the KISS principle, try to write simple code. Think of many solutions for your problem, then choose the best, simplest one and transform that into your code. Whenever you find lengthy code, divide that into multiple methods. If we have some functionality written by one developer and it was written with messy code, and if we ask for another developer to make modifications in that code, then first, they have to understand the code. Obviously, if the code is written simply, then there will not be any difficulty in understanding that code, and also will be easy to modify.