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.
String object can not be changeed,but StringBuffer object can. When you want to change a String object( Example: use method replace('a','b')),in fact, it create a new String Object and return it to you.