About CALLBACK : ---
Callback is a piece of executable code that is passed as an
argument to other code, which is expected to call back (execute) the argument
at some convenient time.
![alt text](./etc/callback.png "Callback")
Use the Callback pattern when
* when some arbitrary synchronous or asynchronous action must be performed after execution of some defined activity.
## Real world examples
* [CyclicBarrier](http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CyclicBarrier.html#CyclicBarrier%28int,%20java.lang.Runnable%29) constructor can accept callback that will be triggered every time when barrier is tripped.