System.out.println("18. Memento: ");
HelloWorldMementoOriginator helloWorldMementoOriginator = new HelloWorldMementoOriginator();
HelloWorldMementoOriginator.Memento memento = helloWorldMementoOriginator.set("Hello Memento!").saveToMemento();
helloWorldMementoOriginator.set("Hello Whatever!");
helloWorldMementoOriginator.restoreFromMemento(memento);
System.out.println(helloWorldMementoOriginator.helloWorld());
System.out.println("19. Observer: ");
Subject subject = new Subject().attach(new HelloWorldObserver());
subject.notifyObservers();