Observer (Design Pattern)

Main.java

import java.util.List;
import java.util.ArrayList;
import java.util.Iterator;

class Subject {
  private List<Observer> observers = new ArrayList<Observer>();
  
  public void addObserver(Observer observer) {
    observers.add(observer);
  }
  
  public void removeObserver(Observer observer) {
    observers.remove(observer);
  }
  
  public void notifyObservers() {
    Iterator it = observers.iterator();
    while (it.hasNext()) {
      Observer obs= (Observer) it.next();
      obs.update(this); 
    }
  }
  
}

interface Observer {
  public void update(Subject s);
}


class Temperatura extends Subject  {
   private double temp;
   
   public double getTemp() {
     return temp;
   }
   public void setTemp(double temp) {
     this.temp = temp;
     notifyObservers();
   }
}

class TermometroCelsius implements Observer  {
  public void update(Subject s) {
    double temp = ((Temperatura) s).getTemp();
    System.out.println("Temperatura Celsius: " + temp);
  }
}

public class Main {
  
   public static void  main(String [] args) {
      Temperatura t = new Temperatura();
      t.addObserver(new TermometroCelsius ());
      t.setTemp(100.0);
   }

}