Instructions
Carefully read these instructions before starting.
This test requires the use of Java.  If you're not proficient in Java, please contact us to arrange an alternative test.
Feel free to use any resources at your disposal.
Your code should be of production quality.  Please test it thoroughly, and include appropriate documentation for anything that isn't obvious.
This exercise mandates subclassing/inheritance, which admittedly is not the most flexible solution. Feel free to put comments in the code explaining other designs that you would have explored and their advantages/disadvantages.
Java SE 5 or later is recommended, along with its features such as Generics and Enums.  Please do not use a release earlier than the 1.4.2 JDK.
If you know a testing framework such as JUnit or TestNG, please implement your tests using such a framework.  If not, include a main method in at least one class that implements your tests.
If you know Maven, feel free to implement your exercise as a Maven project.  If you don't, don't worry about it.
 
Functional Requirements for "ObservableList"
 
Create a new class ObservableList that extends java.util.ArrayList
ObservableList must allow any class that implements a new interface (that you define) called ListListener to "register" and "unregister" with it.
All ListListeners that are registered with an ObservableList must be "notified" every time one or more items are added or removed from that list, no matter what method was called to modify the list. Be sure to check the API docs for ArrayList to make sure you cover all of the various add and remove methods.
The notification must allow the listeners to access the items that were added/removed and the List they belong to.
It is up to you to decide how to best implement the notification mechanism, however for this exercise notifications only need to be sent to objects that live in the same JVM process. In other words, don't use JMS or anything distributed.
There are no specific requirements on how this class should deal with multiple concurrent threads. Implement the best, most reasonable multi-threading features you can think of given the scope of this exercise. At a minimum, please document in your code comments all of the important multi-threaded issues you can think of.
Create a class TestListListener that implements ListListener and prints every notification it receives from ObservableList. It should print at least what kind of change occurred and the item(s) that were added/removed.
Include at least one unit test that uses ObservableList with TestListListener and demonstrates its features.  (You're more than welcome to include more.)  It should use at least two instances of ObservableList and at least three instances of TestListListener (you can use more). Use whatever combinations of lists and listeners and method calls you think best demonstrates the most functionality.