An immutable object is one whose state cannot change; it represents a constant of some kind. java.lang.Integer, Long, Short, Character, Byte, Float, and Double are some of the other immutable classes in the JDK. There are others, and of course anyone can write an immutable class.
My mate went for a job interview in java and they asked him how "they could change a string by passing it into a method". ANSWER they can't, or use a StringBuffer [ November 27, 2003: Message edited by: Steve Roderick ]