why String class is immutable?

[saikrishna cinux]

why javasoft need to keep Sting class immutable?
and why StringBuffer is mutable?

[William Brogden]

The Java designers recognized that many many security holes are created by sloppy handling of strings. Therefore they made modification of an existing string variable impossible. This also has other advantages - for example the intern() method saves storage space.
Bill

[Jeff Albertson]

...and given that String is immutable, the reason for the existence of StringBuffer should be obvious -- sometimes you want to be able to alter a sequence of characters.