List<?> or List<Object>

Question

I have a method whose argument should be "a List of anything". The method will not modify the contents of the List. Is it more correct to define this method as

void foo(List<?> list) {
}

or

void foo(List<Object> list) {
}

and what exactly is the difference?

Answer 1

Short answer, using List<?> will allow you to accept something like List<String> while using List<Object> won't.

This is discussed in the official generics trail, here and here.

[...] here is a naive attempt at writing it using generics (and the new for loop syntax):

   void printCollection(Collection<Object> c) {
       for (Object e : c) {
           System.out.println(e);
       }
   }

The problem is that this new version is much less useful than the old one. Whereas the old code could be called with any kind of collection as a parameter, the new code only takes Collection<Object>, which, as we've just demonstrated, is not a supertype of all kinds of collections!
So what is the supertype of all kinds of collections? It's written Collection<?> (pronounced "collection of unknown"), that is, a collection whose element type matches anything. It's called a wildcard type for obvious reasons. We can write:

   void printCollection(Collection<?> c) {
       for (Object e : c) {
           System.out.println(e);
       }
   }

and now, we can call it with any type of collection.

Answer 2

The List<Object> has parameterized type Object which essentially means that you can add all objects in the list.

However, List<?> means that is a list of unknowns. As long as you're not going to add anything to the list, it's essentially the same as List<Object> but you will have to worry with circumstances as follows:

List<?> unknownList = new ArrayList<String>();
unknownList.add(new Object()); //Compilation error.

The ? (in this case) means that you can only add values of String or is subtype of String.

If you're just going to iterate through the list and retrieve values, List<?> and List<Object> is essentially the same.

More info here.