Unless a method is explicitly intended to mutate an object received as an argument, you should think twice before aliasing the argument object by simply assigning it to an instance variable in your class. If in doubt, make a copy. Your clients will be happier. Of course, making a copy is not free: there is a cost in CPU and memory. However, an API that causes subtle bugs is usually a bigger problem than one that is a little slower or uses more resources.
