Design Patterns

Composite

interface Components{
  void showPrice();
}

class Composite implements Components{
  String name;
  List<Components> componentsList = new ArrayList<>();
  
  public Composite(String name){
    super();
    this.name = name;
  }

  public void addComponents(Components c){
	  componentsList.add(c);
  }

	@Override
	public void showPrice(){
      System.out.println(name);
      for(comp c : componentsList){
        c.showPrice();
      }
   }
}


class Leaf implements Components{

int price;
String name;

public Leaf(int price, String name){
  this.price = price;
  this.name = name;
}

  @Override
	public void showPrice(){
      System.out.println(name + " : " + price);
   }
}

public static void main(){
  Components hardDrive = new Leaf(4000, "HardDrive");
  Components mouse = new Leaf(500, "Mouse");
  Components monitor = new Leaf(300, "Monitor");
  Components ram = new Leaf(5000, "RAM");
  Components cpu = new Leaf(800, "CPU");
  
  Composite peripheral = new Composite("Peripheral");
  Composite cabinet = new Composite("Cabinet");
  Composite motherBoard = new Composite("MotherBoard");
  Composite computer = new Composite("Computer");                  //root 
  
  peripheral.addComponents(mouse);
  peripheral.addComponents(monitor);
  
  cabinet.addComponents(hardDrive);
  
  motherBoard.addComponents(cpu);
  motherBoard.addComponents(ram);
  
  cabinet.addComponents(motherBoard);

  computer.addComponents(peripheral);
  computer.addComponents(cabinet);
  
  ram.showPrice();                      //5000 
  peripheral.showPrice();             
  //Peripheral
  //Mouse : 500
  //Monitor : 300
  
}

so with the single method showPrice() we can calculate price of single leaf & composite node with its leaf

Decorator

interface TextViewComponent {
    void draw();
}

class ConcreteTextView implements TextViewComponent {
    @Override
    public void draw() {
        System.out.println("Drawing basic TextView");
    }
}

// Decorator class
abstract class TextViewDecorator implements TextViewComponent {
    protected TextViewComponent textViewComponent;

    public TextViewDecorator(TextViewComponent textViewComponent) {
        this.textViewComponent = textViewComponent;
    }

    @Override
    public void draw() {
        textViewComponent.draw(); // Delegate drawing to the wrapped component
    }
}

// Concrete decorators
class BorderDecorator extends TextViewDecorator {
    public BorderDecorator(TextViewComponent textViewComponent) {
        super(textViewComponent);
    }

    @Override
    public void draw() {
        super.draw();
        // Add border functionality
        System.out.println("Adding border to TextView");
    }
}

class BackgroundColorDecorator extends TextViewDecorator {
    public BackgroundColorDecorator(TextViewComponent textViewComponent) {
        super(textViewComponent);
    }

    @Override
    public void draw() {
        super.draw();
        // Add background color functionality
        System.out.println("Adding background color to TextView");
    }
}

// Usage example
public class Main {
    public static void main(String[] args) {
        // Create a basic TextView
        TextViewComponent textView = new ConcreteTextView();

        // Decorate it with border
        textView = new BorderDecorator(textView);

        // Decorate it with background color
        textView = new BackgroundColorDecorator(textView);

        // Draw the decorated TextView
        textView.draw();
    }
}

Facade

// Facade class for the music player subsystems
public class MusicPlayerFacade {
    private MediaPlayer mMediaPlayer;
    private AudioManager mAudioManager;
    private NotificationManager mNotificationManager;

    public MusicPlayerFacade(Context context) {
        mMediaPlayer = new MediaPlayer();
        mAudioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
        mNotificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
    }

    public void playMusic(String musicUrl) {
        try {
            // Prepare media player with the provided music URL
            mMediaPlayer.setDataSource(musicUrl);
            mMediaPlayer.prepare();
            mMediaPlayer.start();

            // Request audio focus
            mAudioManager.requestAudioFocus(null, AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN);

            // Show notification
            showNotification("Now playing...");

        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void stopMusic() {
        // Stop media player
        if (mMediaPlayer.isPlaying()) {
            mMediaPlayer.stop();
            mMediaPlayer.release();
        }

        // Abandon audio focus
        mAudioManager.abandonAudioFocus(null);

        // Cancel notification
        mNotificationManager.cancel(1);
    }

    private void showNotification(String message) {
        // Code to create and display a notification
        Notification notification = new Notification.Builder(mContext)
                .setContentTitle("Music Player")
                .setContentText(message)
                .setSmallIcon(R.drawable.ic_music_note)
                .build();

        mNotificationManager.notify(1, notification);
    }
}

Visitor

// Visitor interface
interface ConfigurationChangeListener {
    void onOrientationChange();
    void onLanguageChange();
}

class ConfigurationChangeHandler implements ConfigurationChangeListener {
    @Override
    public void onOrientationChange() {
        // React to orientation change
    }

    @Override
    public void onLanguageChange() {
        // React to language change
    }
}

// ViewModel class
public class MyViewModel extends ViewModel {

    private ConfigurationChangeListener configurationChangeListener;

    // Method to set the configuration change listener
    public void setConfigurationChangeListener(ConfigurationChangeListener listener) {
        this.configurationChangeListener = listener;
    }

    // Method to be called when a configuration change occurs
    public void onConfigurationChange(Configuration newConfig) {
        if (configurationChangeListener != null) {
            if (newConfig.orientation != Configuration.ORIENTATION_UNDEFINED) {           //<---------visitor pattern
                configurationChangeListener.onOrientationChange();
            }
            if (!newConfig.locale.equals(Locale.getDefault())) {                          //<---------visitor pattern
                configurationChangeListener.onLanguageChange();
            }
            // Add conditions for other types of configuration changes
        }
    }
}