Facade Pattern from Head First Design Patterns

In this post, we will learn Facade Pattern with examples from Head First Design Patterns Book.
Table of contents
  1. Overview
  2. Class Diagram
  3. Implementation with Example
  4. Conclusion (Source code on Github Repository)
  5. References

1. Overview

Facade Pattern defined
The Facade Pattern provides a unified interface to a set of interfaces in a subsystem.
Facade defines a higher-level interface that makes the subsystem easier to use.
The Principle of Least Knowledge
Design Principle
The Principle of Least Knowledge - talk only to your immediate friends
The Principle of Least Knowledge guides us to reduce the interaction between objects to just a few close "friends."

2. Class Diagram

Home Theater Class Diagram

3. Implementation with Example


Home Theater is a more complex subsystem so apply Facade Pattern which makes the flexible and unified interface for interaction.
Let's build a Home Theater example from Head First Design Patterns Book.
Here Facade class treats the home theater components as a subsystem and calls the subsystem to implement its watch movie method.
Let's write source code step by step to implement Home Theater
Step 1: Create an Amplifier java class.
public class Amplifier {
 String description;
 Tuner tuner;
 DvdPlayer dvd;
 CdPlayer cd;
 
 public Amplifier(String description) {
  this.description = description;
 }
 
 public void on() {
  System.out.println(description + " on");
 }
 
 public void off() {
  System.out.println(description + " off");
 }
 
 public void setStereoSound() {
  System.out.println(description + " stereo mode on");
 }
 
 public void setSurroundSound() {
  System.out.println(description + " surround sound on (5 speakers, 1 subwoofer)");
 }
 
 public void setVolume(int level) {
  System.out.println(description + " setting volume to " + level);
 }

 public void setTuner(Tuner tuner) {
  System.out.println(description + " setting tuner to " + dvd);
  this.tuner = tuner;
 }
  
 public void setDvd(DvdPlayer dvd) {
  System.out.println(description + " setting DVD player to " + dvd);
  this.dvd = dvd;
 }
 
 public void setCd(CdPlayer cd) {
  System.out.println(description + " setting CD player to " + cd);
  this.cd = cd;
 }
 
 public String toString() {
  return description;
 }
}
Step 2: Create CdPlayer java class.
public class CdPlayer {
 String description;
 int currentTrack;
 Amplifier amplifier;
 String title;
 
 public CdPlayer(String description, Amplifier amplifier) {
  this.description = description;
  this.amplifier = amplifier;
 }
 
 public void on() {
  System.out.println(description + " on");
 }
 
 public void off() {
  System.out.println(description + " off");
 }

 public void eject() {
  title = null;
  System.out.println(description + " eject");
 }
 
 public void play(String title) {
  this.title = title;
  currentTrack = 0;
  System.out.println(description + " playing \"" + title + "\"");
 }

 public void play(int track) {
  if (title == null) {
   System.out.println(description + " can't play track " + currentTrack + 
     ", no cd inserted");
  } else {
   currentTrack = track;
   System.out.println(description + " playing track " + currentTrack);
  }
 }

 public void stop() {
  currentTrack = 0;
  System.out.println(description + " stopped");
 }
 
 public void pause() {
  System.out.println(description + " paused \"" + title + "\"");
 }
 
 public String toString() {
  return description;
 }
}
Step 3: Create DvdPlayer java class.
public class DvdPlayer {
 String description;
 int currentTrack;
 Amplifier amplifier;
 String movie;
 
 public DvdPlayer(String description, Amplifier amplifier) {
  this.description = description;
  this.amplifier = amplifier;
 }
 
 public void on() {
  System.out.println(description + " on");
 }
 
 public void off() {
  System.out.println(description + " off");
 }

        public void eject() {
  movie = null;
                System.out.println(description + " eject");
        }
 
 public void play(String movie) {
  this.movie = movie;
  currentTrack = 0;
  System.out.println(description + " playing \"" + movie + "\"");
 }

 public void play(int track) {
  if (movie == null) {
   System.out.println(description + " can't play track " + track + " no dvd inserted");
  } else {
   currentTrack = track;
   System.out.println(description + " playing track " + currentTrack + " of \"" + movie + "\"");
  }
 }

 public void stop() {
  currentTrack = 0;
  System.out.println(description + " stopped \"" + movie + "\"");
 }
 
 public void pause() {
  System.out.println(description + " paused \"" + movie + "\"");
 }

 public void setTwoChannelAudio() {
  System.out.println(description + " set two channel audio");
 }
 
 public void setSurroundAudio() {
  System.out.println(description + " set surround audio");
 }
 
 public String toString() {
  return description;
 }
}
Step 4: Create Projector java class.
public class Projector {
 String description;
 DvdPlayer dvdPlayer;
 
 public Projector(String description, DvdPlayer dvdPlayer) {
  this.description = description;
  this.dvdPlayer = dvdPlayer;
 }
 
 public void on() {
  System.out.println(description + " on");
 }
 
 public void off() {
  System.out.println(description + " off");
 }

 public void wideScreenMode() {
  System.out.println(description + " in widescreen mode (16x9 aspect ratio)");
 }

 public void tvMode() {
  System.out.println(description + " in tv mode (4x3 aspect ratio)");
 }
  
        public String toString() {
                return description;
        }
}
Step 5: Create TheaterLights java class.
public class TheaterLights {
 String description;
 
 public TheaterLights(String description) {
  this.description = description;
 }
  
 public void on() {
  System.out.println(description + " on");
 }
  
 public void off() {
  System.out.println(description + " off");
 }
  
 public void dim(int level) {
  System.out.println(description + " dimming to " + level  + "%");
 }
   
        public String toString() {
                return description;
        }
}
Step 6: Create Screen java class.
public class Screen {
 String description;
 
 public Screen(String description) {
  this.description = description;
 }
 
 public void up() {
  System.out.println(description + " going up");
 }
 
 public void down() {
  System.out.println(description + " going down");
 }
 
  
        public String toString() {
                return description;
        }
}
Step 7 : Create PopcornPopper java class.
public class PopcornPopper {
 String description;
 
 public PopcornPopper(String description) {
  this.description = description;
 }
 
 public void on() {
  System.out.println(description + " on");
 }
 
 public void off() {
  System.out.println(description + " off");
 }

 public void pop() {
  System.out.println(description + " popping popcorn!");
 }
 
  
        public String toString() {
                return description;
        }
}
Step 8: Let's create Facade Java class - HomeTheaterFacade.
Now it's time to bring components of the subsystem together into a unified interface.
Let's implement watchMovie() and endMovie() methods.
public class HomeTheaterFacade {
 Amplifier amp;
 Tuner tuner;
 DvdPlayer dvd;
 CdPlayer cd;
 Projector projector;
 TheaterLights lights;
 Screen screen;
 PopcornPopper popper;
 
 public HomeTheaterFacade(Amplifier amp, 
     Tuner tuner, 
     DvdPlayer dvd, 
     CdPlayer cd, 
     Projector projector, 
     Screen screen,
     TheaterLights lights,
     PopcornPopper popper) {
 
  this.amp = amp;
  this.tuner = tuner;
  this.dvd = dvd;
  this.cd = cd;
  this.projector = projector;
  this.screen = screen;
  this.lights = lights;
  this.popper = popper;
 }
 
 public void watchMovie(String movie) {
  System.out.println("Get ready to watch a movie...");
  popper.on();
  popper.pop();
  lights.dim(10);
  screen.down();
  projector.on();
  projector.wideScreenMode();
  amp.on();
  amp.setDvd(dvd);
  amp.setSurroundSound();
  amp.setVolume(5);
  dvd.on();
  dvd.play(movie);
 }
 
 
 public void endMovie() {
  System.out.println("Shutting movie theater down...");
  popper.off();
  lights.on();
  screen.up();
  projector.off();
  amp.off();
  dvd.stop();
  dvd.eject();
  dvd.off();
 }

 public void listenToCd(String cdTitle) {
  System.out.println("Get ready for an audiopile experence...");
  lights.on();
  amp.on();
  amp.setVolume(5);
  amp.setCd(cd);
  amp.setStereoSound();
  cd.on();
  cd.play(cdTitle);
 }

 public void endCd() {
  System.out.println("Shutting down CD...");
  amp.off();
  amp.setCd(cd);
  cd.eject();
  cd.off();
 }

 public void listenToRadio(double frequency) {
  System.out.println("Tuning in the airwaves...");
  tuner.on();
  tuner.setFrequency(frequency);
  amp.on();
  amp.setVolume(5);
  amp.setTuner(tuner);
 }

 public void endRadio() {
  System.out.println("Shutting down the tuner...");
  tuner.off();
  amp.off();
 }
}
Step 9: Now it's time to watch a movie. It's SHOWTIME !!.
Now we test the Facade Design Pattern.
public class HomeTheaterTestDrive {
 public static void main(String[] args) {
  Amplifier amp = new Amplifier("Top-O-Line Amplifier");
  Tuner tuner = new Tuner("Top-O-Line AM/FM Tuner", amp);
  DvdPlayer dvd = new DvdPlayer("Top-O-Line DVD Player", amp);
  CdPlayer cd = new CdPlayer("Top-O-Line CD Player", amp);
  Projector projector = new Projector("Top-O-Line Projector", dvd);
  TheaterLights lights = new TheaterLights("Theater Ceiling Lights");
  Screen screen = new Screen("Theater Screen");
  PopcornPopper popper = new PopcornPopper("Popcorn Popper");
 
  HomeTheaterFacade homeTheater = 
    new HomeTheaterFacade(amp, tuner, dvd, cd, 
      projector, screen, lights, popper);
 
  homeTheater.watchMovie("Raiders of the Lost Ark");
  homeTheater.endMovie();
 }
}
Output :
Get ready to watch a movie...
Popcorn Popper on
Popcorn Popper popping popcorn!
Theater Ceiling Lights dimming to 10%
Theater Screen going down
Top-O-Line Projector on
Top-O-Line Projector in widescreen mode (16x9 aspect ratio)
Top-O-Line Amplifier on
Top-O-Line Amplifier setting DVD player to Top-O-Line DVD Player
Top-O-Line Amplifier surround sound on (5 speakers, 1 subwoofer)
Top-O-Line Amplifier setting volume to 5
Top-O-Line DVD Player on
Top-O-Line DVD Player playing "Raiders of the Lost Ark"
Shutting movie theater down...
Popcorn Popper off
Theater Ceiling Lights on
Theater Screen going up
Top-O-Line Projector off
Top-O-Line Amplifier off
Top-O-Line DVD Player stopped "Raiders of the Lost Ark"
Top-O-Line DVD Player eject
Top-O-Line DVD Player off

4. Conclusion

In this post, we have learned the Facade Pattern from Head First Design Patterns book. There is a separate post for Facade Pattern in detail with examples, advantages, real-world examples.
All the source code for this post available on Github Repo. This is eclipse project so you can import into your workspace and play with it.

5. Reference

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