Not sure I agree with the GP's point, but I believe he's arguing that when fulfilling the contract of a fictional MouseClickListener:

  interface MouseClickListener {
    void mouseClicked(MouseEvent e);
  }

one often uses an anonymous class to do so:

  void init() {
    mouse.setClickListener(new MouseClickListener() {
      void mouseClicked(MouseEvent e) {
        println(e);
      }
    });
  }

the exact effect could be achieved with a reference to a named function (if Java supported them, which presumably it will once "everything is an object"):

  void init() {
    void mouseClick(MouseEvent e) { println(e); }
    mouse.setClickListener(mouseClick);
  }

given that interfaces are simply syntax for function routing, when you have the ability to reference functions by first-class types (i.e. you have the ability to determine function routing yourself), the set of things that you can only sensibly do with interfaces is a lot smaller.

This is how C# delegates work, right? Any Java -> C# programmer want to comment on whether they rely less on interfaces now and what they use them for?