Difference between ? (wildcard) and Type Parameter in Java

Question

Can somebody explain me what the difference is between these two methods? Are they same? They do look same to me in terms of what they solve. If they are same, why need

Answer 1

In your examples, there is absolutely no difference at all. Each will produce the same output.

The best use and interpretation of generics requires that you know the semantics of the type parameter as well as something about the parameter's role. The reason for this is that in a case such as your first example above (unbounded wild card) the semantics are "a list of objects of unknown type" and a "parameter that will produce (not consume) List instances.

The method above simply produces each List object and calls toString() on each. All objects are guaranteed to have a toString() method and so it isn't necessary to know anything at all about the object's type for this purpose. This is exactly why the unbounded wildcard is the best choice for this method parameter: To produce List instances and to call toString() on them, it is not necessary to know anything about the object's type.

Note that if the ? had the same semantics ("a List of objects of unknown type") but a different purpose ("the List would consume objects of unknown type") things would change very, very radically such that it may be inadvisable (or very difficult) to use a wildcard parameter (at least without a helper method to capture the object's type).

It is generally not possible to assert one generic parameter form for all situations. The best form to use (wildcard vs. concrete; bounded vs. unbounded; extends vs. super) depends on both the semantics of the type parameter and what the role of the parameter in the method will be.