Should we use WeakReference in Observer Pattern?

I'm learning and trying to implement the Observer pattern in Java from this tutorial. I'm stuck at the point about whether I should use WeakReference for the subject to hold the observers.

For the implementation below, the Subject and the BinaryObserver are holding strong references to each other, which I'm afraid prevents GC from collecting the unused Subject.

So my question is, should I use List<WeakReference<Observer>> observers instead of List<Observer> observers?

Subject.java

public class Subject {

   private List<Observer> observers = new ArrayList<Observer>();
   private int state;

   public int getState() {
      return state;
   }

   public void setState(int state) {
      this.state = state;
      notifyAllObservers();
   }

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

   public void notifyAllObservers(){
       for (Observer observer : observers) {
           observer.update();
       }
   }
}

Observer.java

public abstract class Observer {
    protected Subject subject;
    public abstract void update();
}

BinaryObserver.java

public class BinaryObserver extends Observer{

    public BinaryObserver(Subject subject){
        this.subject = subject;
        this.subject.attach(this);
    }

    @Override
    public void update() {
        System.out.println("Binary String: " + Integer.toBinaryString());
    }
}

ObserverPatternDemo.java

public class ObserverPatternDemo {
    public static void main(String[] args) {
       Subject subject = new Subject();

       new BinaryObserver(subject);

       System.out.println("First state change: 15");
       subject.setState(15);
    }
}