added memento example

src/learn/simple_memento/CareTaker.java

@@ -0,0 +1,19 @@
+package learn.simple_memento;
+
+import java.util.Stack;
+
+public class CareTaker {
+    private final Stack<Originator.Memento> snapshots = new Stack<>();
+
+    public void save(Originator.Memento memento) {
+        snapshots.push(memento);
+    }
+
+    public void revert(Originator originator) {
+        if (!snapshots.isEmpty()) {
+            originator.setMemento(snapshots.pop());
+        } else {
+            System.out.println("No saved memento to revert");
+        }
+    }
+}

src/learn/simple_memento/Main.java

@@ -0,0 +1,19 @@
+package learn.simple_memento;
+
+public class Main {
+    public static void main(String[] args) {
+        Originator originator = new Originator();
+        CareTaker careTaker = new CareTaker();
+
+        originator.setState("blue");
+        originator.setState("red");
+        careTaker.save(originator.createMemento());
+        originator.setState("yellow");
+        careTaker.save(originator.createMemento());
+        originator.setState("white");
+
+        careTaker.revert(originator);
+        careTaker.revert(originator);
+        careTaker.revert(originator);
+    }
+}

src/learn/simple_memento/Originator.java

@@ -0,0 +1,35 @@
+package learn.simple_memento;
+
+public class Originator {
+    private String state;
+
+    public String getState() {
+        return state;
+    }
+
+    public void setState(String state) {
+        this.state = state;
+        System.out.println("[Originator] Setting state to " + state);
+    }
+
+    public Memento createMemento() {
+        return new Memento(state);
+    }
+
+    public void setMemento(Memento memento) {
+        state = memento.getSavedState();
+        System.out.println("[Originator] State after restoring from memento: " + state);
+    }
+
+    public class Memento {
+        private final String state;
+
+        public Memento(String state) {
+            this.state = state;
+        }
+
+        public String getSavedState() {
+            return state;
+        }
+    }
+}