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.
SPACE ( " " ) , which you have used in your Code is treated as a "Empty String" which is still a valid Java Object for the Compiler. But null is not a valid Java Object which can not be stored in ArrayList and Vector.