added observer example

src/learn/simple_observer/ConcreteObserver.java

@@ -0,0 +1,15 @@
+package learn.simple_observer;
+
+public class ConcreteObserver implements Observer {
+    private ConcreteSubject subject;
+
+    public ConcreteObserver(ConcreteSubject subject) {
+        this.subject = subject;
+        subject.registerObserver(this);
+    }
+
+    @Override
+    public void update() {
+        System.out.println("Observer was notified: subject has new state: " + subject.getState());
+    }
+}

src/learn/simple_observer/ConcreteSubject.java

@@ -0,0 +1,37 @@
+package learn.simple_observer;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class ConcreteSubject implements Subject {
+    private List<Observer> observers = new ArrayList<>();
+    int state;
+
+    @Override
+    public void registerObserver(Observer o) {
+        observers.add(o);
+    }
+
+    @Override
+    public void removeObserver(Observer o) {
+        observers.remove(o);
+    }
+
+    @Override
+    public void notifyObservers() {
+        for (Observer o : observers) {
+            o.update();
+        }
+    }
+
+    public void setState(int state) {
+        this.state = state;
+        if (state > 50) {
+            notifyObservers();
+        }
+    }
+
+    public int getState() {
+        return state;
+    }
+}

src/learn/simple_observer/Main.java

@@ -0,0 +1,12 @@
+package learn.simple_observer;
+
+public class Main {
+    public static void main(String[] args) {
+        ConcreteSubject subject = new ConcreteSubject();
+        Observer observer = new ConcreteObserver(subject);
+        subject.setState(40);
+        subject.setState(100);
+        Observer observer2 = new ConcreteObserver(subject);
+        subject.setState(200);
+    }
+}

src/learn/simple_observer/Observer.java

@@ -0,0 +1,5 @@
+package learn.simple_observer;
+
+public interface Observer {
+    void update();
+}

src/learn/simple_observer/Subject.java

@@ -0,0 +1,7 @@
+package learn.simple_observer;
+
+public interface Subject {
+    void registerObserver(Observer o);
+    void removeObserver(Observer o);
+    void notifyObservers();
+}