Mutable cause issue - http://www.artima.com/lejava/articles/equality.html

Very detail discussion about implementing equals - http://www.angelikalanger.com/Articles/JavaSolutions/SecretsOfEquals/Equals.html http://www.angelikalanger.com/Articles/JavaSolutions/SecretsOfEquals/Equals-2.html

Example of implement and TEST equals - http://blog.springfuse.com/2009/10/testing-various-equals-and-hashcode_3597.html

Consistent of equals and compareTo - http://blog.joda.org/2012/11/pitfalls-of-consistent-with-equals.html