Functors and Comparators:  A Summary

• A functor (“function object”) is a class that exists to encapsulate a method. (In many other languages, functions can be defined outside of classes, making functors unnecessary.)

• The java.util.Comparator interface is one that is often implemented as a functor.

  • Use java.util.Collections.sort(List<T>, Comparator<? super T>) to sort objects. This is useful for objects that lack a compareTo method, or to sort them in a way other than what that method defines.

  • A Comparator’s compare(T o1, To2) method returns:

    int ≤ -1

    When o1 < o2.

    0

    When o1 equals o2

    int ≥ 1

    When o1 > o2

• This ActionListener is basically a functor, implemented as an anonymous class.

  quitButton.addActionListener(new ActionListener() {
  	public void actionPerformed(ActionEvent e) {
  		System.exit(0);
  	}
  });