Java Inheritance, how does a extending a class effect the actual class

I am reviewing the Sun Certification study guide and there is a passage which describes the final modifier. It says

"If programmers were free to extend the String class civilisation as we know it could collapse"

What does he mean ? If it were possible to extend String Class ... would I just not have a class called MyString which inherits all of the Strings properties. How would it be possible to change the actual String class in any way by only extending it ?

Thank you very much for your answers