просто

public interface Subject {
public void attach(Observer observer);
public void detach(Observer observer);
public void notifyObservers();
}

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

public class HelloWorldSubject implements Subject {

private List<Observer> observers;
private String str;

public HelloWorldSubject() {
super();

observers = new ArrayList<Observer>();
}

public void attach(Observer observer) {
observers.add(observer);
}

public void detach(Observer observer) {
observers.remove(observer);
}

public void notifyObservers() {
Iterator<Observer> iter = observers.iterator();

while (iter.hasNext()) {
Observer observer = iter.next();
observer.update(this);
}
}

public String getStr() {
return str;
}

public void setStr(String str) {
this.str = str;
notifyObservers();
}
}

public class HelloWorldObserver implements Observer {

public void update(Subject subject) {
HelloWorldSubject sub = (HelloWorldSubject)subject;
System.out.println(sub.getStr());
}

}


public interface Command {
void execute();
}

public class HelloWorldCommand implements Command {

private HelloWorldSubject subject;

public HelloWorldCommand(Subject subject) {
super();

this.subject = (HelloWorldSubject)subject;
}

public void execute() {
subject.setStr("hello world");
}

}

public interface AbstractFactory {
public Subject createSubject();
public Observer createObserver();
public Command createCommand(Subject subject);
}

public class HelloWorldFactory implements AbstractFactory {

public Subject createSubject() {
return new HelloWorldSubject();
}

public Observer createObserver() {
return new HelloWorldObserver();
}

public Command createCommand(Subject subject) {
return new HelloWorldCommand(subject);
}
}


public class FactoryMakerSingleton {

private static FactoryMakerSingleton instance = null;
private AbstractFactory factory;

private FactoryMakerSingleton() {
factory = new HelloWorldFactory();
}

public static synchronized FactoryMakerSingleton getInstance() {
if (instance == null) {
instance = new FactoryMakerSingleton();
}

return instance;
}

public AbstractFactory getFactory() {
return factory;
}
}

public class AbuseDesignPatterns {

public static void main(String[] args) {
AbstractFactory factory = FactoryMakerSingleton.getInstance().getFactory();

Subject subject = factory.createSubject();
subject.attach(factory.createObserver());

Command command = factory.createCommand(subject);

command.execute();
}

}