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.
To sort things, you need to compare them. Can you compare Integers with Strings or Booleans ? Look at the API for Arrays#sort(Object). That's what they mean by "all elements in the array must be mutually comparable".