@Override
        public void run() {
            r.run();
            currentSize--;
        }
Facebook interview question
Implement a non block run which takes as many runnables as given but only
runs a specified number of threads.
You can also update maxSize in between which will change number of threads executing
http://tutorials.jenkov.com/java-concurrency/thread-pools.html
src/com/interview/multithreaded/ThreadPoolImpl.java