It is complicated...
For any type variable T, the spec says http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.4
Every type variable ... has a bound. If no bound is declared for a type variable, Object is assumed.
One would think that it's true for wildcard too, and ? should just be a shorthand for ? extends Object.
Yet searching through the spec, there is no evidence at all that a wildcard must have an upper bound (or lower bound). The "unbounded" ? is treated consistently distinctly from bounded wildcards.
We could deduce from subtyping rules, that List and List are subtypes of each other, i.e., they are basically the same type. (The deduction depends on the fact that E in interface List has an implicit upper bound Object; but the rules do not require bounds on wildcards)
Nevertheless the spec treats the two differently. For example http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.7 List is a reifiable type, but List is not, which means
// ok
List[] xx = {};
// fail
List[] yy = {};
// ok
boolean b1 = (y instanceof List);
// fail
boolean b2 = (y instanceof List);
I don't understand why though. It seems perfectly fine to say a wildcard must have an upper bound and a lower bound, default to Object and null type.
