This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Normally not, but classes like String and the primitive wrappers (Integer, Boolean, etc) are immutable. That means that once created they can never change. It is therefore safe to share them among multiple objects, methods and threads without any problems.