There is a new class in Java5.0 - StringBuilder class that can be used whenever we are doing some heavy-duty string manipulation and we do not wish to get bogged down by the immutable property of strings.
But then what happened to the old good StringBuffer class. Well, it's still there.
Here's what the JavaDoc says:
This StringBuilder class provides an API compatible with StringBuffer, but with no guarantee of synchronization. This class is designed for use as a drop-in replacement for StringBuffer in places where the string buffer was being used by a single thread (as is generally the case). Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.
Instances of StringBuilder are not safe for use by multiple threads. If such synchronization is required then it is recommended that StringBuffer be used.