Nathan,
It's a tricky one to be sure.
The problem is that the entrySet() method is returning a
"Set<Map.Entry<capture-of ? extends K, capture-of ? extends V>>",
which is incompatible with the type "Set<Map.Entry<? extends K, ?
extends V>>". It's easier to describe why if I drop the "extends K"
and "extends V" part. So we have "Set<Map.Entry<?, ?>" and
"Set<Map.Entry<capture-of ?, capture-of ?>>".
The first one, "Set<Map.Entry<?, ?>>" is a set of Map.Entries of
different types - ie it is a heterogeneous collection. It could
contain a Map.Entry<Long, Date> and a Map.Entry<String, ResultSet>>
and any other pair of types, all in the same set.
On the other hand, "Set<Map.Entry<capture-of ?, capture-of ?>>" is a
homogenous collection of the same (albeit unknown) pair of types. Eg
it might be a Set<Map.Entry<Long, Date>>, so all of the entries in the
set MUST be Map.Entry<Long, Date>.
In general, even if you could assign this to a local variable without
unchecked type safety warnings, you wouldn't be able to call any
methods on it because the compiler cannot check whether the access is
type-safe. For example you would not be able to call add() because the
Set must contain a specific type, but the compiler doesn't know which
specific type that is!
Regards
Neil
On 5/11/06, Nathan Beyer <[EMAIL PROTECTED]> wrote:
Does someone understand why this works this way? This seems so odd. I know
there are quirks to the generics syntax, but this in an edge I haven't run
into yet. I haven't been able to make this one click in my head yet.
This compiles fine:
public synchronized void putAll(Map<? extends K,? extends V> map) {
for (Map.Entry<? extends K,? extends V> entry : map.entrySet()) {
...
}
}
This won't compile at all:
public synchronized void putAll(Map<? extends K,? extends V> map) {
Set<Map.Entry<? extends K, ? extends V>> entries = map.entrySet();
...
}
The error is: Type mismatch: cannot convert from Set<Map.Entry<capture-of ?
extends K,capture-of ? extends V>> to Set<Map.Entry<? extends K,? extends
V>>
The suggested quick fix in Eclipse is "Set<?> entries = ...".
This reports a warning:
public synchronized void putAll(Map<? extends K,? extends V> map) {
Map<K,V> map2 = (Map<K,V>)map;
}
The warning is: Type safety: The cast from Map<capture-of ? extends
K,capture-of ? extends V> to Map<K,V> is actually checking against the
erased type Map
The suggested quick fix in Eclipse is to the annotation:
@SuppressWarnings("unchecked").
-Nathan
> -----Original Message-----
> From: Tim Ellison [mailto:[EMAIL PROTECTED]
> Sent: Wednesday, May 10, 2006 6:59 AM
> To: harmony-dev@incubator.apache.org
> Subject: Thanks Stepan! (was: Re: [jira] Resolved: (HARMONY-454)
> [classlib][luni] java.util.Set generics uplift and related changes)
>
> Stepan Mishura (JIRA) wrote:
> <snip>
> > 2) To avoid casting while-loop was replaced with for-loop. Could you
> review the change?
>
> I was scratching my head about this cast, so I was very pleased to see
> your elegant solution.
>
> I must admit that I don't really understand why the for-loop version is
> inherently different (other than it 'hides' the casting) -- but I've
> learned a new pattern there :-)
>
> Regards,
> Tim
>
> --
>
> Tim Ellison ([EMAIL PROTECTED])
> IBM Java technology centre, UK.
>
>
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Neil Bartlett
Senior Technical Consultant, Integility Ltd
Tel: +44 (0) 20 7043 8328
Fax: +44 (0) 20 7043 8329
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