How to achieve strong immutability for an immutable object?
i. make your class as final, to achieve strong immutability to your object.
ii. Make your class all constructors as private, to avoid others from inheriting it, and there by you can achieve strong immutability for your object.
iii. Just make all fields as private and final, don't return any references, and don't give any setters to private variables, then it will become strong immutable object.