code: public static void setNumberOfThreads(int numThreads) {
  		if (numThreads < 1)
  			throw new IllegalStateException("Number of threads must be a positive integer.");
  		
  		if (threadPool.size() > numThreads)
  			throw new IllegalStateException("You cannot set a numThreads smaller than the current size of the threads pool.");
  		
  		if (threadPool.size() < numThreads) {
  			for (int i=threadPool.size(); i<numThreads; i++) {
  				threadPool.add(Executors.newSingleThreadExecutor());
  			}
  		}
  	}
docs: /*
  Create and manage threads