No cast for observer pattern using generics

Question

I was looking to implement an subject-observer pattern where the subject provides its self to the observers when notifying.

public class Subject<T extends Subject> {

    /** suporting stuff for subject */

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

    protected void doNotify() {
        for(Observer<T> observer : observers) {
            /** This is the line where it gets interesting */
            observer.update((T)this);
        }
    }
}

Practically, this work, however, the compiler gives a Unchecked cast warning on the observer.update((T)this); line.

When reading a bit about this, the compiler is right (surprise surprise) and its even considered as smelly code, as you can write code that actually triggers the ClassCastException.

Now I am looking for a solution that isn't smelly and rock solid. However, the thought that an observer does not need to look for the subject that it is observing is something I really like. Also, I don't really like the observers need to do the cast themselves in their update(). Do you have any suggestion on how to on this one?

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Edit

My observer is declared as interface like this:

public interface Observer<T> {
    void update(T subject);
}

Answer 1

Foreword: I would suggest NOT to use generics- because by forcing the clients (Observers) to know about the exact type of a Subject, (as opposed to having a non-generic Subject class), you cannot eg. subscribe the same Observer to multiple Subjects (of different types).

Anyway, there is a way to have type-safety .

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In the call observer.update((T)this) you want two things: you want to pass this to observers; and you also want for this to be of type T.

At that point, this is not guaranteed to be of type T of course- it is of type Subject. But "this" will be of type T in a concrete Subject class. So substitute this with getThisSubject() and move it down in the hierarchy. In code:

package stackOv;
import java.util.*;

abstract class Subject<T extends Subject<T>> {
  private List<Observer<T>> observers = new ArrayList<>();
  // returns a reference of this, of type T
  protected abstract T getThisSubject();
  protected void doNotify() {
    for(Observer<T> observer : observers) {
      observer.update(getThisSubject());
    }
  }
  public void addObserver(Observer<T> obs) {
    observers.add(obs);
  }
}

class SubjectA extends Subject<SubjectA> { 
  @Override
  protected SubjectA getThisSubject() {
    return this;
  }
}

interface Observer<T> {
  void update(T subject);
}

class UseSubject {
  public static void main(String[] args) {
    SubjectA sub = new SubjectA();

    Observer<SubjectA> obs = new Observer<SubjectA>() {
      @Override
      public void update(SubjectA subject) {
        //and here we have a reference to the right type
        System.out.println("subj=" + subject);
      }
    };

    sub.addObserver(obs);
    sub.doNotify();
  }
}

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Let me stress that we are coupling Observer types with Subject types; they really are in a one-to-one relationship. A way to avoid this is declaring Subject non-generic, and

interface Observer {
  void update(Subject subject);
}

plus using visitor or other patterns. (but this reply is long enough already).