Interface to interface association in the book Head First Design Patterns

Question

The book Head First Design Patterns presents the following UML as an example of the Observer pattern:

What strikes me in this diagram is the association relationship between the Subject and Observer interfaces. As far as I understand Java interfaces, they cannot implement a "Has-a" relationship in this way.

When I look at the implementation example provided a few pages later, I find that sure enough, the interfaces are plain old interfaces:

public interface Subject {
  public void registerObserver(Observer o);
  public void removeObserver(Observer o);
  public void notifyObservers;
}

public interface Observer {
  public void update(float temp, float humidity, float pressure);
}

Instead, the association relationship is implemented in the concrete WeatherData and Display classes:

public class WeatherData implements Subject {
  private List<Observer> observers;
  //more code
}

Which does not correspond to the UML and feels a bit wrong to me. Why not just implement Subject as an abstract class instead? Does the UML spec formalize the idea of associating an interface with another interface?