List<Object> and List<?>

Why cant I do this:

List<Object> object = new List<Object>();

You can't do this because List is an interface, and interfaces cannot be instantiated. Only (concrete) classes can be. Examples of concrete classes implementing List include ArrayList, LinkedList etc.

Here is how one would create an instance of ArrayList:

List<Object> object = new ArrayList<Object>();

I have a method that returns a List<?>, how would I turn that into a List<Object>

Show us the relevant code and I'll update the answer.

List<Object> object = new List<Object>();

You cannot do this because List is an interface and you cannot create object of any interface or in other word you cannot instantiate any interface. Moreover, you can assign any object of class which implements List to its reference variable. For example you can do this:

list<Object> object = new ArrayList<Object>();

Here ArrayList is a class which implements List, you can use any class which implements List.

List is an interface so you can't instanciate it. Use any of its implementatons instead e.g.

List<Object> object = new List<Object>();

About List : you can use any object as a generic param for it instance:

List<?> list = new ArrayList<String>();

or

List<?> list = new ArrayList<Integer>();

While using List<Object> this declaration is invalid because it will be type missmatch.