Java Reference
In-Depth Information
Here, we have
A
=
G
<? super
V
>
<<
G
<? extends
U
>
. In general, we cannot con-
clude anything in this case. However, it is not necessarily an error. It may be
that
U
will eventually be inferred to be
Object
, in which case the call may in-
deed be valid. Therefore, we simply refrain from placing any constraint on
U
.
• If
F
has the form
G
<
...,
Y
k-1
,
? super
U
,
Y
k+1
, ...
>
, where
U
involves
T
j
, then if
A
has
a supertype that is one of:
♦
G
<
...,
X
k-1
,
V
,
X
k+1
, ...
>
. Then this algorithm is applied recursively to the con-
straint
V
>>
U
.
As usual, we consider only the case where
G
has a single type argument.
A
<<
F
in this case means
A
<<
G
<? super
U
>
. As above, it must be the case
that
A
is a subtype of some invocation of
G
.
A
may now be a subtype of either
G
<
V
>
, or
G
<? extends
V
>
, or
G
<? super
V
>
. We examine these cases in turn. The
first variation is described (generalized to multiple arguments) by the sub-bul-
let directly above. We therefore have
A
=
G
<
V
>
<<
G
<? super
U
>
. The rules of
subtyping for wildcards imply that
V
>>
U
.
♦
G
<
...,
X
k-1
,
? super
V
,
X
k+1
, ...
>
. Then this algorithm is applied recursively to the
constraint
V
>>
U
.
We have
A
=
G
<? super
V
>
<<
G
<? super
U
>
. The rules of subtyping for lower-
bounded wildcards again imply that
V
>>
U
.
♦ Otherwise, no constraint is implied on
T
j
.
Here, we have
A
=
G
<? extends
V
>
<<
G
<? super
U
>
. In general, we cannot con-
clude anything in this case. However, it is not necessarily an error. It may be
that
U
will eventually be inferred to the null type, in which case the call may
indeed be valid. Therefore, we simply refrain from placing any constraint on
U
.
• Otherwise, no constraint is implied on
T
j
.
3.
Otherwise, if the constraint has the form
A
=
F
:
• If
F
=
T
j
, then the constraint
T
j
=
A
is implied.
• If
F
=
U
[]
where the type
U
involves
T
j
, then if
A
is an array type
V
[]
, or a type
variable with an upper bound that is an array type
V
[]
, where
V
is a reference type,
this algorithm is applied recursively to the constraint
V
=
U
.
If the array types
U
[]
and
V
[]
are the same, their component types must be the
same.
