Patron Observer

src/main/java/org/example/designpatterns/observer/App.java

+package org.example.designpatterns.observer;
+
+import org.example.designpatterns.observer.observer.PesoARGObserver;
+import org.example.designpatterns.observer.observer.PesoMXOberserver;
+import org.example.designpatterns.observer.observer.SolObserver;
+import org.example.designpatterns.observer.observer.Subject;
+
+public class App {
+
+    public static void main(String[] args) {
+        Subject subject = new Subject();
+
+        new SolObserver(subject);
+        new PesoARGObserver(subject);
+        new PesoMXOberserver(subject);
+
+        System.out.println("Si desea cambiar 10 dólares obtendrá: ");
+        subject.setEstado(10);
+        System.out.println("-------------------");
+        System.out.println("Si desea cambia 100 dlares obtendrá: ");
+        subject.setEstado(100);
+    }
+}

src/main/java/org/example/designpatterns/observer/observer/Observer.java

+package org.example.designpatterns.observer.observer;
+
+public abstract class Observer {
+    protected Subject subject;
+    public abstract void actualizar();
+}

src/main/java/org/example/designpatterns/observer/observer/PesoARGObserver.java

+package org.example.designpatterns.observer.observer;
+
+public class PesoARGObserver extends Observer{
+    private double valorCambio = 29.86;
+
+    public PesoARGObserver(Subject subject) {
+        this.subject = subject;
+        this.subject.agregar(this);
+    }
+
+    @Override
+    public void actualizar() {
+        System.out.println("ARG: " + (subject.getEstado() * valorCambio));
+    }
+}

src/main/java/org/example/designpatterns/observer/observer/PesoMXOberserver.java

+package org.example.designpatterns.observer.observer;
+
+public class PesoMXOberserver extends Observer{
+    private double valorCambio = 19.07;
+
+    public PesoMXOberserver(Subject subject) {
+        this.subject = subject;
+        this.subject.agregar(this);
+    }
+
+    @Override
+    public void actualizar() {
+        System.out.println("MX: " + (subject.getEstado() * valorCambio));
+    }
+}

src/main/java/org/example/designpatterns/observer/observer/SolObserver.java

+package org.example.designpatterns.observer.observer;
+
+public class SolObserver extends Observer{
+
+    private double valorCambio = 3.75;
+
+    public SolObserver(Subject subject) {
+        this.subject = subject;
+        this.subject.agregar(this);
+    }
+
+    @Override
+    public void actualizar() {
+        System.out.println("PEN: " + (subject.getEstado() * valorCambio));
+    }
+}

src/main/java/org/example/designpatterns/observer/observer/Subject.java

+package org.example.designpatterns.observer.observer;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Subject {
+    private List<Observer> observers = new ArrayList<>();
+    private int estado;
+
+    public int getEstado() {
+        return estado;
+    }
+
+    public void setEstado(int estado) {
+        this.estado = estado;
+        notificarTodosObservadores();
+    }
+
+    public void agregar(Observer observer) {
+        observers.add(observer);
+    }
+
+    private void notificarTodosObservadores() {
+        for (Observer observer: observers) {
+            observer.actualizar();
+        }
+    }
+}