How can I use the Observer Pattern for file monitoring with threads?

Question

I am trying to implement the observer pattern to a game i have made. When a villain is created in the battle-zone file using threads, I would like to use the observer pattern to create a hero using threads and add it to the same file. The villians and heroes are created using the factory method pattern. I am unsure of where to go with regards to linking my HeroCreationMain class to the observer pattern classes.

Villian Creation

public class VillianCreationMain {

private static Villian villian;

public static void main(String[] args, int userInput) throws IOException {

    String fileName = null;

    Random randomVillian = new Random();

    int amountOfVillians = userInput;

    if (amountOfVillians < 7) {

        for (int x = 0; x < amountOfVillians; x++) {

            int randomGenerator = randomVillian.nextInt(6);

                for (int i = 0; i < 5; i++) {

                    if (randomGenerator == 0 ) {

                        setVillian(new FlyingVillian());

                    }

                    else if (randomGenerator == 1) {

                        setVillian(new StrongVillian());

                    }

                    else if (randomGenerator == 2) {

                        setVillian(new FastVillian());

                    }

                    else if (randomGenerator == 3) {

                        setVillian(new SmartVillian());

                    }

                    else if (randomGenerator == 4) {

                        setVillian(new FireVillian());

                    }

                    else if (randomGenerator == 5) {

                        setVillian(new IceVillian());

                    }

                        try {

                            writeToFile(getVillian(), i, fileName);
                        }
                        catch (IOException e) {

                            System.out.println(e.getMessage());
                        }

                    }

                VillianThreads t1 = new VillianThreads(VillianCreationMain.getVillian());

                t1.start();

                }

            }

    else {

            System.out.println("Please enter a value of less than 7");
    }
}

public static void writeToFile(Villian villian, int amountOfVillians, String fileName) throws IOException {

    for(int x = 0; x < amountOfVillians; x++) {

    //  String parsedInt = Integer.toString(x);

        fileName = "battle-zone.ser";

        FileOutputStream file = new FileOutputStream(fileName);
        ObjectOutputStream oos = new ObjectOutputStream(file);

        oos.writeObject(villian);

        file.close();
        oos.close();
    }
}

public static Villian getVillian() {
    return villian;
}

public static void setVillian(Villian villian) {
    VillianCreationMain.villian = villian;
}

}

Hero Creation

public class HeroCreationMain {

private static Hero hero = null;

public static void main(String[] Hero) {

    EnemyStatus enemyStatus = new EnemyStatus();

    VillianObserver observer1 = new VillianObserver(enemyStatus);

}

public static void readFile() throws IOException, ClassNotFoundException {

    @SuppressWarnings("resource")
    ObjectInputStream ois = new ObjectInputStream (new FileInputStream("battle-zone.ser"));

    Villian targetVillian = (Villian) ois.readObject();
    System.out.println(targetVillian + " is being attacked by a hero!");
}

public static Hero getHero() {
    return hero;
}

public static void setHero(Hero hero) {
    HeroCreationMain.hero = hero;
}
}

Observer

public interface Observer {

public void update(boolean enemyPresent);

}

public interface Subject {

public void register(Observer o);
public void unregister(Observer o);
public void notifyObserver();

}

Observable

public class VillianObserver implements Observer {

private boolean enemyPresent;
private static int heroIDTracker;
private int heroID;
private Subject villianObserver;

public VillianObserver(Subject villianObserver) {

    this.villianObserver = villianObserver;
    this.heroID = ++heroIDTracker;
    System.out.println("New Observer " + this.heroID);
    villianObserver.register(this);
}

@Override
public void update(boolean enemyPresent) {

    this.enemyPresent = enemyPresent;
    printResult();
}

public void printResult() {

    System.out.println(heroID + " " + enemyPresent);
}

}

Enemy Status

import java.util.ArrayList;

public class EnemyStatus implements Subject {

private ArrayList<Observer> observers;
private boolean enemyPresent;

public EnemyStatus() {

    // Creates an ArrayList to hold all observers

    observers = new ArrayList<Observer>();

}

@Override
public void register(Observer newObserver) {

    observers.add(newObserver);

}

@Override
public void unregister(Observer deleteObserver) {

    // Get the index of the observer to delete

    int heroIndex = observers.indexOf(deleteObserver);

    // Print out message (Have to increment index to match  

    System.out.println("Observer " + (heroIndex+1) + " deleted");        

    // Removes observer from the ArrayList

    observers.remove(heroIndex);


}

@Override
public void notifyObserver() {

    for(Observer observer : observers) {

        observer.update(enemyPresent);
    }

}

public void setEnemyStatus(boolean enemyPresent) {

    this.enemyPresent = enemyPresent;
    notifyObserver();

}

}  

Answer 1

JNotify is the Java library to observe file changes on the file system.

One piece of advice: Object(Input/Output)Streams are easy when you're just getting started but they lead you down a path of ruin. Objects get so easily BadVersion'ed. Object files are also relatively hard to inspect using a text editor. I'd advise you to try using a different data format (like JSON) instead.