不变性immutable

http://www.javapractices.com/topic/TopicAction.do?Id=29

Immutable objects have a very compelling list of positive qualities. Without question, they are among the simplest and most robust kinds of classes you can possibly build. When you create immutable classes, entire categories of problems simply disappear.

Make a class immutable by following these guidelines:

ensure the class cannot be overridden - make the class final, or use static factories and keep constructors private
make fields private and final
force callers to construct an object completely in a single step, instead of using a no-argument constructor combined with subsequent calls to setXXX methods (that is, avoid the Java Beans convention)
do not provide any methods which can change the state of the object in any way - not just setXXX methods, but any method which can change state
if the class has any mutable object fields, then they must be defensively copied when they pass between the class and its caller

Defensive copying:
http://www.javapractices.com/topic/TopicAction.do?Id=15
http://www.manning-sandbox.com/message.jspa?messageID=30035

Collections is not immutable, so Encapsulate collections
http://www.javapractices.com/topic/TopicAction.do?Id=173