Why does the compiler treat List and List differently?

Question

Consider the following code:

import java.util.ArrayList;
import java.util.List;

public class UnboundedWildcardProblem {

    public static void main(String[] args) {
        List<?> a = new ArrayList();
        List<? extends Object> b = new ArrayList();
    }
}

The creation of List<?> doesn't produce any warnings, but the creation of List<? extends Object> produces an unchecked warning:

Warning: java: unchecked conversion
  required: java.util.List<? extends java.lang.Object>
  found:    java.util.ArrayList

I searched for possible reasons and found some discussions:

List<?> vs List<? extends Object>

What's the difference between <?> and <? extends Object> in Java Generics?

But after reading these discussions it seems to me that List<?> and List<? extends Object> are the same thing.

So, what is the reason for such compiler behaviour?

EDIT:

The single flag used for compilation was an Xlint:unchecked flag.

I mean... aren't those different generic declarations? `List>` seems different from `List extends Object>` — Caffeinated, Nov 18 '15 at 22:02
[Don't use raw types](http://stackoverflow.com/questions/2770321/what-is-a-raw-type-and-why-shouldnt-we-use-it). This has very little to do with the declared type of `a` and `b`. — Sotirios Delimanolis, Nov 18 '15 at 22:03
"The creation of List> doesn't produce any warnings". Wrong, it does, because "ArrayList is a raw type." — Tunaki, Nov 18 '15 at 22:05
I dont know if this is true, but shouldnt `? extends Object` exclude Object and provide only objects that extend from it? — Kaloyan Roussev, Nov 18 '15 at 22:05
Nope, I am wrong. If a extends b, a is considered to be substitutable for b — Kaloyan Roussev, Nov 18 '15 at 22:07
I don't know but the second declaration has a bounds (`extends`) so it might be treated differently internally by the compiler. The main thing here is "don't use raw types" so that's really the root of the issue here. — markspace, Nov 18 '15 at 22:08
The first snippet DOES produce a warning, contrary to what the OP says. — Tunaki, Nov 18 '15 at 22:09
[JLS 5.1.9. Unchecked Conversion](https://docs.oracle.com/javase/specs/jls/se8/html/jls-5.html#jls-5.1.9) — rgettman, Nov 18 '15 at 22:13
How are you compiling your code? Are you instructing the compiler to show _all_ warnings using `-Xlint`? — M A, Nov 18 '15 at 22:15
@manouti i compile with -Xlint:unchecked to show all unchecked warnings. — Edgar Rokjān, Nov 18 '15 at 22:17
Recompile with "-Xlint:rawtypes": http://stackoverflow.com/questions/697879/javac-flag-to-disallow-raw-types — Tunaki, Nov 18 '15 at 22:22

score 10 · Accepted Answer · edited Jun 20 '20 at 09:12

The creation of List<?> doesn't produce any warnings, but the creation of List<? extends Object> produces an unchecked warning

In fact, if you compile your code with the -Xlint:all or just Xlint option, you would get a warning for the List<?> a = new ArrayList(); statement, something like:

   warning: [rawtypes] found raw type: ArrayList
            List<?> a = new ArrayList();
                            ^
missing type arguments for generic class ArrayList<E>
where E is a type-variable:
  E extends Object declared in class ArrayList

However, the reason why you're not getting an unchecked warning per se is because the type List<?> contains only unbounded wildcards as mentioned in this section from the Java Language Specification:

There is an unchecked conversion from the raw class or interface type (§4.8) G to any parameterized type of the form G<T1,...,Tn>.

...

Use of an unchecked conversion causes a compile-time unchecked warning unless all type arguments Ti (1 ≤ i ≤ n) are unbounded wildcards (§4.5.1), or the unchecked warning is suppressed by the SuppressWarnings annotation (§9.6.4.5).

However in this section, it is mentioned:

The wildcard ? extends Object is equivalent to the unbounded wildcard ?.

which shows a subtle inconsistency in the compiler.

Thanks for the detailed explanation! – Edgar Rokjān Nov 18 '15 at 22:37 — Edgar Rokjān, Nov 18 '15 at 22:37

Why does the compiler treat List and List differently?

1 Answers1