Consider this case:
class A {}
class B<T extends A, E extends T> {
B<?, A> b;
B<?, ? extends A> b2;
}
As I understand type bounds, in this case effective upper bounds of both T
and E
is class A
. So the question: why javac doesn't accept class A as argument in declaration of field b
, but accepts wildcard
? extends A
in declaration of field b2
?
With the following classes:
class A {}
class C extends A {}
class B<T extends A, E extends T> {}
Think of it like this:
E extends T extends A
With B<?,A>
then T -> ?
and E -> A
A extends ? extends A
Where the ?
could be any subclass of A
, let's say C
.
A extends C extends A
is clearly invalid.
So that's why it's a compile error.
Eclipse 4.9.0 compiler disagreed with javac 8u and Intellij and did not emit a compile error for the generic arguments in B<?,A>
. I assume this is a bug in the Eclipse compiler but I have not consulted the JLS to confirm this.
class B<T extends A, E extends T> {
B<?, A> b; // <-- Eclipse does NOT emit a compile error
B<?, ? extends A> b2;
}
This assumed bug has been reported here.