This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
String class implements immutable character strings
String is immutable. String keeps an array of characters which is mutable as any other array. But String don't have a method to modify this array and restrict subclassing String class by declaring the class final.
The interface String implements is CharSequence which is not mandating the implementor should be immutable.