designPatterns

The main goal of this project is to show basic design patterns and provide real-life examples.
Solutions provided in the code are inspired by the wide-ranging internet search and know-how of the author.
Valuable references:
http://www.blackwasp.co.uk/GofPatterns.aspx
http://www.oodesign.com/
Most of design patterns are given with the example from Java SE.

behavioural

All behavioural design patterns are in package: behavioural.*.
Package of specific pattern is obtained using camel-case.
Every design pattern has been tested in test package (same naming convention).

chain of responsibility

• theory: chain of responsibility is used to process varied requests, each of which may be dealt with by a different handler. The most usual example of a machine using the chain of responsibility is the vending machine coin slot: rather than having a slot for each type of coin, the machine has only one slot for all of them.
• code: We have Request with size field, and handlers extending Handler (with Handler field successor): HandlerFirstImpl process requests of positive size otherwise pushes them to the successor: HandlerSecondImpl, which process requests of negative size (note that at that point, all requests with either size > 0 & size < 0 where handled, so the last case is size = 0) otherwise pushes them to the end-point successor: HandlerThirdImpl.
• Java SE example: java.util.logging.Logger#log()

command

• theory: command is used to encapsulate all information needed to perform an action. Four terms always associated with the command pattern are command, receiver, invoker and client:
  command object knows about receiver and invokes a method of the receiver.
  receiver is stored in the command object by aggregation and does the work when the execute() method in command is called.
  invoker object knows how to execute a command, but does not know anything about a concrete command, it knows only about command interface.
  client holds invoker, command and receiver objects.
• code: We have different platforms (Mac and Windows) and we want to perform action on files (open, write, close). We have platform dependent FileSystemReceiver (interface with methods open, write, close) implementations: MacFileSystemReceiver and WindowsFileSystemReceiver. For invoking commands: FileInvoker.
• Java SE example: all implementations of java.lang.Runnable

interpreter

• theory: interpreter is used to define easily-extendable grammar for instructions that form part of a language or notation.
• code: We have an expression containing words and spaces, and we want to develop a grammar that enables us to answer such questions: check if expression contains specific word LiteralExpression and furthermore we could connect questions with basic binary operators: or (OrExpression) & and (AndExpression) to compose more complex questions like: check if expression contains word1 and (word2 or (word3 or word4)) (SpecificInterpreter).
• Java SE example: java.util.Pattern

mediator

• theory: mediator is used to reduce coupling between classes that communicate with each other by developing mechanism to facilitate the interaction between objects in a manner in which objects are not aware of the existence of other objects.
• code: We have a ChatMediator that facilitates sending messages between Users.
• Java SE example: java.util.concurrent.ExecutorService (submit() methods)

memento

• theory: memento is used to save the current state of an object in such a manner that it can be restored at a later time without breaking the rules of encapsulation.
• code: We have a TextEditor with functionality of addWord(String word). If we make a mistake we call undo().

observer

• theory: In the observer pattern, an object, called the subject, maintains a list of other objects, which are its observers. When the state of the subject changes, its observers are notified.
• code: We have subject Earth with observers: AsteroidObserver and SpaceStationObserver that observe FlyingObject (with appropriate FlyingObjectType) flying near Earth.

strategy

• theory: strategy is used to create an interchangeable family of algorithms from which the required process is chosen at run-time. Most known example of strategy is Collections.sort(List<T> list, Comparator<? super T> c).
• code: We have a ShoppingCart and we pay using CreditCardPayment or PayPalPayment (different algorithms). In the ShoppingCart we have a method pay(Payment method) where we put appropriate payment algorithm.
• Java SE example: java.util.Collections# sort(List<T> list, Comparator<? super T> c)

template method

• theory: template method is used to define an algorithm in a base class using abstract operations that subclasses provide with concrete behavior.

• code: We have a base class Trip with template method performTrip() that consists of:

  1. destinationTransport (start)
  2. daysSchedule (vacations)
  3. homeTransport (end)

  We provide TwoDayTrip extends Trip and we can compose two day trip like:

  TwoDayTrip twoDayTrip = new TwoDayTrip(
          () -> TransportType.TRAIN,
          () -> PlaceType.SEA,
          () -> PlaceType.SEA,
          () -> TransportType.PLANE
  );
  

• Java SE example: java.io.InputStream #read(byte b[], int off, int len)

visitor

• theory: visitor is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existent object structures without modifying the structures.
• code: We have different configurations (LinuxConfigurator, MacConfigurator) of different routers (DLinkRouter, LinkSysRouter) and we could easily add another type configuration (ex. MSConfigurator) without changing code of Routers, and we could add another type of Router by simply modifying code of Configurator (RouterVisitor).
• Java SE example: java.nio.file.SimpleFileVisitor

creational

All creational design patterns are in package: creational.*.
Package of specific pattern is obtained using camel-case.
Every design pattern has been tested in test package (same naming convention).

abstract factory

• theory: abstract factory is an interface for creating families of related or dependent objects without specifying their concrete classes.
• code: We have two possible platforms - Mac and MS, and we have to produce desktop application that runs on either of them. As we know widgets implementation differ on different platforms, so we have to encapsulate production of windows and buttons into PlatformDependentWidgetProducer and in run-time we decide which factory (MsWindowsWidgetFactory or MacOSXWidgetFactory) should be used (it depend on which platform we are).

builder

• theory: builder is an external class that facilitates the construction of an object. Note that we could have two different types of objects (by their ability to change state): immutable and mutable.
• code: For immutable we have: ImmutableWithBuilder - we have private constructor that has a builder argument constructor and we have static class Builder (with same fields that an original class) that have setters that allow chaining.
  For mutable objects we have GenericBuilder (using java 8) that could be used for every mutable class.
• Java SE example: java.lang.StringBuilder#append()

factory method

• theory: factory method defines an interface for creating an object, but leaves the choice of its type to the subclasses, creation being deferred at run-time.
• code: ShapeFactory produces different Shapes depending on ShapeType.
• Java SE example: java.util.ResourceBundle#getBundle()

prototype

• theory: prototype is used to instantiate a new object by copying all of the fields of an existing object (construction of a new object could be time consuming).
• code: We have TimeConsumingCreationBaseFont that construction takes two seconds (TimeUnit.SECONDS.sleep(2) in constructor), but this has also clone() method, so we have to prototype object only once and then copy whenever we need new object (HelveticaFont).
• Java SE example: java.lang.Object#clone() (the class has to implement java.lang.Cloneable)

singleton

• theory: singleton is used when we have to ensure that only one object of a particular class is ever created, and all references refer to the same object.
• code: ThreadSafeSingleton. Note that not every singleton is thread safe, which is common mistake.
• Java SE example: java.lang.System#console()

structural

All structural design patterns are in package: creational.*.
Package of specific pattern is obtained using camel-case.
Every design pattern has been tested in test package (same naming convention).

adapter

• theory: adapter is used when we have two incompatible types and we want to have a communication between them. When one class relies on the interface that the other does not implement - adapter is a translator between them.
• example: We have two incompatible interfaces of the same purpose: one in english (IPerson), one in french (IFrenchPerson) - we integrate these two system into one - using adapter FrenchPersonAdapter that implements IPerson. So we have easy way to transform every Frenchman to Englishman.
• Java SE example: java.util.Arrays#asList()

bridge

• theory: bridge is used when we want to separate abstractions from technical implementation - implementation details could be changed easily then.
• example: We have two different implementations of music player: Spotify and Tidal that differs in technical implementations, but AbstractMusicPlayer that is used as a bridge, provides that we could easy switch between them.

composite

• theory: composite is used when creating hierarchical object models. It is used to provide that the individual objects and groups can be accessed in the same manner.
• example: We have ComplexShape that is a composition of Shapes, and every Shape could be decomposed to BasicShapes.
• Java SE example: java.util.Pattern

decorator

• theory: decorator is used when we want to extend implementation in run-time (as an alternative to inheritance). It is obtain by wrapping classes in an object called decorator.
• example: We have BasicCar implements Car, and we want to have two upgrades: LuxuryCar and SportsCar. Using inheritance will cause a lot of problems - for example wiring dependencies between them (what in the case of another types, for example ConvertibleCar?!), so we decide to use CarDecorator and two appropriate classes that extends it: LuxuryCar, SportsCar. Dependency is decoupled then - we could easily make Luxury-Sport car and Sport-Luxury car (and they are, like in real life, not the same thing!).
• Java SE example: I/O Streams

facade

• theory: facade is used to define a simplified interface to a more complex subsystem. It is also very useful when API is still under construction and change rapidly, and we don't want to make perpetual changes in our code.
• code: We want to book hotel (HotelBooking) and flight (FlightBooking) in one place (TravelBooking). More sophisticated and real life example is in my another project: https://github.com/mtumilowicz/reports#pdf-1 PdfCellBuilder - easy builder for very complex task of building a pdf call.

flyweight

• theory: flyweight is used to reduce the memory (and time consuming creation) usage for complex models using numerous, but the same objects.
• code: We have immutable Font that has three factors: name, size, color. Creation of font could be time consuming task, so it's better to cache already created fonts and reuse them. Simple reports could demand thousands of different fonts. In Font class we have static class FlyweightFactory that is responsible for creating and caching Font object. Note that good practice is that Font is immutable and has private constructor.
• Java SE example: java.lang.Integer#valueOf(int)

proxy

• theory: proxy is a placeholder class that has plenty applications. There are many different kind of proxy, but we decide to show only Cache Proxy. A cache proxy improves the performance of long-running tasks by caching request-answer. Note that answer should be always the same for specific input. Good example are problems with prime numbers.
  Other type of proxy is described here: https://developer.jboss.org/blogs/stuartdouglas/2010/10/12/weld-cdi-and-proxies
• code: We have algorithm from my another project (https://github.com/mtumilowicz/poligon#CountPrimes) that is already optimal and lightning-fast, but we could shorten the time of receiving count using proxy (only first request is evaluated, others are taken from cache).

tests

Coverage: 91%