This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The first one is an empty reference - a null reference if you will. So no, you can't call orderList.add(order); on it because you would get a NullPointerException, (unless you assign an object to it between the variable definition and method call).
The second one actually creates an Object of a suitable, concrete implementation of List, and so can have a method called on it without an error occurring.