Java's official documentation says:

It's important to note that List<Object> and List<?> are not the same. You can insert an Object, or any subtype of Object, into a List<Object>. But you can only insert null into a List<?>.

Reference link.

This does not seem to be true. These two compile well and are usable (e.g. the list elements can be printed just fine):

List<Object> l1 = Arrays.asList(null, null);
List<?> l2 = Arrays.asList(null, null);
l1.add(null);
l2.add(null);

Could you please say what the quoted sentence in the documentation really means?

CodePudding user response：

No, the text is correct. null is a valid value for any type, therefore, you can invoke .add(null) on any list, regardless of what is in the <?>. However, null (literally) is the only thing you can add to a List<? extends Whatever>. Given that literally .add(null) is generally useless, let's disregard it, which lets us simply state:

You cannot add anything to a List<?>.

That's the point of what the text is trying to explain.

Why can't you add anything? Because of this:

Let's say you have a List<? extend Number>. That means you can have a List<Integer>, or a List<Double> - those both 'work'. But given that you don't know what it is, if you just try to add an Integer object to this list (which is a Number), then maybe you're adding an Integer to a list of Doubles which would be an illegal move. Conclusion: .add() on a list of <?> or <? extends something> is not possible. (Except for the academic case of null).

CodePudding user response：

Your test case does not cover the statement: You're not inserting objects, you're creating lists around predefined sets of objects.

Instead try something like

List<Object> l1 = new ArrayList<>();
l1.add(null);
l1.add("foo");

List<?> l2 = new ArrayList<>();
l2.add(null);
l2.add("foo"); // <-- here we expect a compilation error