Wow, every response in the thread has been wrong so far.
You can define a class inside another class.
You cannot instantiate b directly in main() only because b is declared private in a.
If b were declared public in a, then amit0991's response would be right.
good thing I deleted my post ^^, I too said you could make an object of it, until I reread it was private and then wasn't sure so hit the delete button.