The article says:
To make a class immutable you can define its all constructors private and then create a public static method to initialize and object and return it.
Actually, that is wrong. These two concepts aren't really related.
E.g. you could declare the constructor of your Employee class public and it would still be immutable.
Or you could pass a mutable Object as a parameter to the factory method or declare a mutator method
-> Employee would be mutable although you're using a factory method and a private constructor.
