What is the facade design pattern

Question

Is facade a class which contains a lot of other classes    What makes it a design pattern  To me  it is like a normal class    Can you explain to me this Facade pattern

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Facade discusses encapsulating a complex subsystem within a single interface object. This reduces the learning curve necessary to successfully leverage the subsystem. It also promotes decoupling the subsystem from its potentially many clients. On the other hand, if the Facade is the only access point for the subsystem, it will limit the features and flexibility that "power users" may need.

Source: https://sourcemaking.com/design_patterns/facade

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A facade should not be described as a class which contains a lot of other classes  It is in fact a interface to this classes and should make the usage of the classes easier otherwise the facade class is useless

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A facade is a class with a level of functionality that lies between a toolkit and a complete application  offering a simplified usage of the classes in a package or subsystem  The intent of the Facade pattern is to provide an interface that makes a subsystem easy to use     -- Extract from  book Design Patterns in C

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Wikipedia has a great example of Facade pattern      Complex parts     class CPU       public void freeze               public void jump long position              public void execute              class Memory       public void load long position  byte   data             class HardDrive       public byte   read long lba  int size                Facade     class ComputerFacade       private CPU processor      private Memory ram      private HardDrive hd       public ComputerFacade             this processor   new CPU            this ram   new Memory            this hd   new HardDrive               public void start             processor freeze            ram load BOOT ADDRESS  hd read BOOT SECTOR  SECTOR SIZE            processor jump BOOT ADDRESS           processor execute                Client     class You       public static void main String   args            ComputerFacade computer   new ComputerFacade            computer start

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A design pattern is a general reusable solution to a commonly occurring problem within a given context in software design   The Facade design pattern is a structural pattern as it defines a manner for creating relationships between classes or entities  The facade design pattern is used to define a simplified interface to a more complex subsystem   The facade pattern is ideal when working with a large number of interdependent classes  or with classes that require the use of multiple methods  particularly when they are complicated to use or difficult to understand  The facade class is a  wrapper  that contains a set of members that are easily understood and simple to use  These members access the subsystem on behalf of the facade user  hiding the implementation details   The facade design pattern is particularly useful when wrapping subsystems that are poorly designed but cannot be refactored because the source code is unavailable or the existing interface is widely used  Sometimes you may decide to implement more than one facade to provide subsets of functionality for different purposes   One example use of the facade pattern is for integrating a web site with a business application  The existing software may include large amounts of business logic that must be accessed in a particular manner  The web site may require only limited access to this business logic  For example  the web site may need to show whether an item for sale has reached a limited level of stock  The IsLowStock method of the facade class could return a Boolean value to indicate this  Behind the scenes  this method could be hiding the complexities of processing the current physical stock  incoming stock  allocated items and the low stock level for each item

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A short and simple explanation    Facade pattern provides a unified interface to a set of interface s  in a subsystem  Facade defines a higher-level interface that makes the subsystem easier to use    Try to understand the scenario with and without Fa  ade  If you want to transfer the money from accout1 to account2 then the two subsystems to be invoked are  withdraw from account1 and deposit to account2

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Another example of facade  say your application connects to database and display results on the UI  You can use facade to make your application configurable  as in run using database or with mock objects  So you will make all the database calls to the facade class  where it will read app config and decide to fire the db query or return the mock object  this way application becomes db independent in case db is unavailable

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A facade exposes simplified functions that are mostly called and the implementation conceals the complexity that clients would otherwise have to deal with  In general the implementation uses multiple packages  classes and function there in  Well written facades make direct access of other classes rare  For example when I visit an ATM and withdraw some amount  The ATM hides whether it is going straight to the owned bank or is it going over a negotiated network for an external bank  The ATM acts like a facade consuming multiple devices and sub-systems that as a client I do not have to directly deal with

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A design pattern is a common way of solving a recurring problem  Classes in all design patterns are just normal classes  What is important is how they are structured and how they work together to solve a given problem in the best possible way    The Facade design pattern simplifies the interface to a complex system  because it is usually composed of all the classes which make up the subsystems of the complex system    A Facade shields the user from the complex details of the system and provides them with a simplified view of it which is easy to use  It also decouples the code that uses the system from the details of the subsystems  making it easier to modify the system later   http   www dofactory com Patterns PatternFacade aspx  http   www blackwasp co uk Facade aspx  Also  what is important while learning design patterns is to be able to recognize which pattern fits your given problem and then using it appropriately  It is a very common thing to misuse a pattern or trying to fit it to some problem just because you know it  Be aware of those pitfalls while learning using design patterns

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The facade pattern is a wrapper of many other interfaces in a result to produce a simpler interface    Design patterns are useful as they solve recurring problems and in general simplify code  In a team of developers who agree to use the same patterns it improves efficiency and understanding when maintaining each others code   Try reading about more patterns    Facade Pattern  http   www dofactory com Patterns PatternFacade aspx  self1  or more generally  http   www dofactory com Patterns Patterns aspx

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Regarding your queries      Is Facade a class which contains a lot of other classes    Yes  It is a wrapper for many sub-systems in application       What makes it a design pattern  For me  it is like a normal class   All design patterns too are normal classes   Unmesh Kondolikar rightly answered this query      Can you explain me about this Facade  I am new to design patterns    According to GoF   Facade design pattern is defind as    Provide a unified interface to a set of interfaces in a subsystem  Facade Pattern defines a higher-level interface that makes the subsystem easier to use  The Facade pattern is typically used when    A simple interface is required to access a complex system  The abstractions and implementations of a subsystem are tightly coupled  Need an entry point to each level of layered software  System is very complex or difficult to understand    Let s take a real word example of cleartrip website   This website provides options to book   Flights Hotels Flights   Hotels   Code snippet   import java util     public class TravelFacade      FlightBooking flightBooking      TrainBooking trainBooking      HotelBooking hotelBooking       enum BookingType           Flight Train Hotel Flight And Hotel Train And Hotel               public TravelFacade            flightBooking   new FlightBooking            trainBooking   new TrainBooking            hotelBooking   new HotelBooking                      public void book BookingType type  BookingInfo info           switch type               case Flight                     book flight                  flightBooking bookFlight info                   return              case Hotel                     book hotel                  hotelBooking bookHotel info                   return              case Train                     book Train                  trainBooking bookTrain info                   return              case Flight And Hotel                     book Flight and Hotel                 flightBooking bookFlight info                   hotelBooking bookHotel info                   return               case Train And Hotel                     book Train and Hotel                 trainBooking bookTrain info                   hotelBooking bookHotel info                   return                                    class BookingInfo      String source      String destination      Date    fromDate      Date     toDate      List lt PersonInfo gt  list    class PersonInfo      String name      int       age      Address address    class Address     class FlightBooking      public FlightBooking               public void bookFlight BookingInfo info            class HotelBooking      public HotelBooking               public void bookHotel BookingInfo info            class TrainBooking      public TrainBooking               public void bookTrain BookingInfo info              Explanation    FlightBooking  TrainBooking and HotelBooking are different sub-systems of large system   TravelFacade TravelFacade offers a simple interface to book one of below options  Flight Booking Train Booking  Hotel Booking Flight   Hotel booking  Train   Hotel booking  book API from TravelFacade internally calls below APIs of sub-systems  flightBooking bookFlight trainBooking bookTrain info   hotelBooking bookHotel info    In this way  TravelFacade provides simpler and easier API with-out exposing sub-system APIs    Key takeaways     from journaldev article by Pankaj Kumar     Facade pattern is more like a helper for client applications Facade pattern can be applied at any point of development  usually when the number of interfaces grow and system gets complex  Subsystem interfaces are not aware of Facade and they shouldn   t have any reference of the Facade interface Facade pattern should be applied for similar kind of interfaces  its purpose is to provide a single interface rather than multiple interfaces that does the similar kind of jobs   Have a look at sourcemaking article too for better understanding

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One additional use of Fa  ade pattern could be to reduce the learning curve of your team  Let me give you an example   Let us assume that your application needs to interact with MS Excel by making use of the COM object model provided by the Excel  One of your team members knows all the Excel APIs and he creates a Facade on top of it  which fulfills all the basic scenarios of the application  No other member on the team need to spend time on learning Excel API  The team can use the facade without knowing the internals or all the MS Excel objects involved in fulfilling a scenario  Is not it great   Thus  it provides a simplified and unified interface on top of a complex sub-system

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It is basically single window clearance system You assign any work it will delegate to particular method in another class

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I like an example from Eric Freeman  Elisabeth Freeman  Kathy Sierra  Bert Bates - Head First Design Patterns book  Example  let s assume you created home theatre and finally you would like to watch a movie  So you have to do          Amplifier amplifier   new Amplifier            CdPlayer cdPlayer   new CdPlayer            DvdPlayer dvdPlayer   new DvdPlayer            Lights lights   new Lights            PopcornPopper popcornPopper   new PopcornPopper            Projector projector   new Projector            Screen screen   new Screen             popcornPopper turnOn            popcornPopper pop            amplifier turnOn            amplifier setVolume 10           lights turnOn            lights dim 10           screen up            dvdPlayer turnOn            dvdPlayer play     what happens when movie is over  You have to do the same but in reverse order so complexity of watch and end movie is becoming very complex  Facade pattern says that you can create a facade and let user just call one method instead of calling all of this  Let s create facade  public class HomeTheatherFacade       Amplifier amplifier      DvdPlayer dvdPlayer      CdPlayer cdPlayer      Projector projector      Lights lights      Screen screen      PopcornPopper popcornPopper       public HomeTheatherFacade Amplifier amplifier  DvdPlayer dvdPlayer  CdPlayer cdPlayer  Projector projector  Lights lights  Screen screen  PopcornPopper popcornPopper        this amplifier   amplifier      this dvdPlayer   dvdPlayer      this cdPlayer   cdPlayer      this projector   projector      this lights   lights      this screen   screen      this popcornPopper   popcornPopper     public void watchMovie String movieTitle        popcornPopper turnOn        popcornPopper pop        amplifier turnOn        amplifier setVolume 10       lights turnOn        lights dim 10       screen up        dvdPlayer turnOn        dvdPlayer play       public void endMovie         dvdPlayer turnOff        screen down        lights turnOff        amplifier turnOff         and now instead of calling all of this you can just call watchMovie and endMovie methods  public class HomeTheatherFacadeTest       public static void main String   args           Amplifier amplifier   new Amplifier            CdPlayer cdPlayer   new CdPlayer            DvdPlayer dvdPlayer   new DvdPlayer            Lights lights   new Lights            PopcornPopper popcornPopper   new PopcornPopper            Projector projector   new Projector            Screen screen   new Screen                     HomeTheatherFacade homeTheatherFacade   new HomeTheatherFacade amplifier  dvdPlayer  cdPlayer  projector  lights  screen  popcornPopper           homeTheatherFacade watchMovie  quot Home Alone quot            homeTheatherFacade endMovie             So    quot The Facade Pattern provides a unified interface to a set of interfaces in a subsytem  Facade defines a higher level interface that makes the subsystem easier to use  quot

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Facade hides the complexities of the system and provides an interface to the client from where the client can access the system    public class Inventory   public String checkInventory String OrderId        return  Inventory checked        public class Payment   public String deductPayment String orderID        return  Payment deducted successfully         public class OrderFacade   private Payment pymt   new Payment    private Inventory inventry   new Inventory     public void placeOrder String orderId        String step1   inventry checkInventory orderId       String step2   pymt deductPayment orderId       System out              println  Following steps completed     step1                          amp      step2           public class Client          public static void main String args              OrderFacade orderFacade   new OrderFacade             orderFacade placeOrder  OR123456             System out println  Order processing completed

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Facade Design Pattern comes under Structural Design Pattern  In short Facade means the exterior appearance  It means in Facade design pattern we hide something and show only what actually client requires  Read more at below blog  http   www sharepointcafe net 2017 03 facade-design-pattern-in-aspdotnet html

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There is a very good real-life example of the pattern  - The car starter engine   As drivers  we just turn the key on and the car get started  As simple as possible  Behind the scenes  many other car systems are involved  as battery  engine  fuel  etc    in order the car to start successfully  but they are hidden behind the starter   As you can see  the car starter is the Facade  It gives us easy to use interface  without worrying about the complexity of all other car systems   Let s summarize   The Facade pattern simplifies and hides the complexity of large code blocks or APIs  providing a cleaner  understandable and easy of use interface

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Its simply creating a wrapper to call multiple methods   You have an A class with method x   and y   and B class with method k   and z    You want to call x  y  z at once   to do that using Facade pattern you just create a Facade class and create a method lets say xyz    Instead of calling each method  x y and z  individually you just call the wrapper method  xyz     of the facade class which calls those methods   Similar pattern is repository but it s mainly for the data access layer

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As explained in the previous answer it provides a simple interface to the consuming client  For example   watch ESPN  is the intended function  But it involves several steps like    Switch on TV if required  Check for satellite cable functioning  Switch to ESPN if required    But the facade will simplify this and just provide  watch ESPN  function to the client

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All design patterns are some classes arranged in some way or other that suits a specific application  The purpose of facade pattern is to hide the complexity of an operation or operations  You can see an example and learn facade pattern from http   preciselyconcise com design patterns facade php

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The facade pattern provides a unified interface to the subsystem interface group  The facade defines a high-level interface  which simplifies the work with the subsystem

[design-patterns] What is the facade design pattern?

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