OnSubscribe.java

Observable.create(OnSubscribe<T> onSubscribe);

public interface OnSubscribe<T> {
  void call(Subscriber<T> subscriber);
}

public abstract class Subscriber<T> 
implements Observer<T>, Subscription { /**/ }

public interface Observer<T> {
  void onCompleted();
  void onError(Throwable e);
  void onNext(T t);
}

public interface Subscription {
  void unsubscribe();
  boolean isUnsubscribed();
}