Created Saturday 07 September 2013

1. Make the class final
2. make all members final, set them explicitly, in a static block, or in the constructor
3. Make all members private
4. No Methods that modify state
5. Be extremely careful to limit access to mutable member components (remember the field may be final but the object can still be mutable. ie private final Date imStillMutable) - See defensive copying or its cousin copy constructors for more info.

(1) is very important and easy to overlook. If its not final people can freely extend your class, override behavior, then supply their new class as a suitable substitute. By declaring the class final you can ensure it will never be extended. This is indicative of the problems surrounding inheritance especially when it is applied inappropriately.

Links

• http://stackoverflow.com/questions/6305752/how-to-create-immutable-objects-in-java