StringBuilder class

String classes create Strings that are immutable in other words when you use the String class to work with strings, the string has a fixed length and you can't edit the characters that make up the string. If you assigne a new value to a string variable, the original String object is deleted an it's replaced with a new String object that contains the new value. StringBuilder was introduced with Java 1.5 with more flexibililty and is mutable, meaning you can add, delete or replace the characters in a StringBuilder object.

http://docs.oracle.com/javase/tutorial/java/data/buffers.html