DefaultMethods

From APIDesign

DefaultMethods is a new feature in JDK8 which breaks the clear separation between Java interface (only specifies a contract) and class (provides some implementation). Many members of the Java community were crying for having a way to add methods into interface in a backward compatible way for ages. Of course, as usual inJava, only when the JDK team felt the need itself (because of adding a lot of new methods into Collection & co. classes), it listen to the general request.

On the other hand, there were people claiming that DefaultMethods are bad - that an interface should be a code-less specification and the change will have consequences. Here is one.

By-Passing Your Interface

When I was implementing Html4Java API, I had to create an observable list - so I did it and created JSONList. I've carefully overwritten each public method that modified the list state and called a change notification handler. What can be the problem?

@Override
public boolean add(T e) {
  boolean ret = super.add(e);
  notifyChange();
  return ret;
}

I thought I did everything correctly, as all the methods of the List interface are properly overwritten. But of course, the problem appeared with DefaultMethods. If you write this code:

People p = new People();
List<String> names = p.getNicknames();
Collections.sort(names);

it works properly on JDK7, but it gets broken on JDK8: the notification change isn't delivered!