I have a class that gets in its constructor a list of objects, List<Object>. Each time the list could be made out of elements from a different type. It's something generic, and I don't know what their class type will be.
I want to save my self a copy of that list, before letting the user change its values. But since the copy is done by reference, both lists (original and copy) are being changed...
How can I copy my list by value?
Your question is not clear.
If you want a shallow copy (i.e. a copy of the list that will contain references to the objects in the original list, then the clone method will do the job, as will a copy constructor on a List implementation class.
If you want a deep copy (i.e. a copy of the list containing copies of the original objects) then your best bet is to create a new list and populate it with clones of the original list elements. However, there is a catch. The clone method is not provided by default. A lot of common utility classes are cloneable, by custom classes are not ... unless you've implemented it. If your list contains any non-cloneable object, you will get exceptions at runtime.
There are other deep-copying alternatives to clone, but just like cloning they don't work with all classes. And really, that's the crux of the problem if your solution has to be generic.