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.
Exception in thread "main" java.lang.NullPointerException
The stacktrace gives the full details. On line 19 of the HundredGame.java file, in the startGame() method is code, which when called gives a null pointer exception. A null pointer exception is generally thrown when you are trying to dereference (ie. use) an object which hasn't been instantiated.