Re: Unchecked Cast

From:

Nigel Wade <nmw@ion.le.ac.uk>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 30 Nov 2007 14:15:42 +0000

Message-ID:

<fip5uf$qh9$1@south.jnrs.ja.net>

KDawg44 wrote:

On Nov 29, 12:48 pm, KDawg44 <KDaw...@gmail.com> wrote:

On Nov 29, 12:42 pm, KDawg44 <KDaw...@gmail.com> wrote:

Hi,

I am getting an error when I compile my app that i hoping someone can
shed some light on. I am working with OpenChord and am getting a
Serialized object. I am trying to cast that to a RegistrationInfo
Object in my program. I did this by:

                try {
                        return
(RegistrationInfo)chord.retrieve(retrieveKey);
                } catch (ServiceException e) {
                        throw new RuntimeException(e);
                }

When I go to compile, I first get this:

Note: PresenceServer.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

(PresenceServer.java is the class the above code is in).

So I compile with the -Xlint option and I get this:

PresenceServer.java:102: warning: [unchecked] unchecked cast
found : java.util.Set<java.io.Serializable>
required: ClientPackage.RegistrationInfo
                        return
(RegistrationInfo)chord.retrieve(retrieveKey);
                                                               ^
1 warning

What am I doing wrong?

Thanks for any suggestions.

This appears to be just a warning so can I ignore it for the time
being? What causes this?

Thanks.

Okay... so there must be a reason for a warning.... This is now
causing my application to break....

        public RegistrationInfo lookup(String username) {
                StringKey retrieveKey = new StringKey(username);
                try {
                        return (RegistrationInfo)
chord.retrieve(retrieveKey);
                } catch (ServiceException e) {
                        throw new RuntimeException(e);
                }

        }

Exception:
java.lang.ClassCastException: java.util.HashSet
        at ClientPackage.PresenceServer.lookup(PresenceServer.java:
107)
        at ClientPackage.ChatClient.talk(ChatClient.java:47)
        at ClientPackage.SendMessage.run(SendMessage.java:55)
        at java.lang.Thread.run(Thread.java:595)

The reason for the warning is to cover exactly this eventuality, attempting to
cast to RegistrationInfo an object which isn't of the class RegistrationInfo.
The methodology your code is using has taken responsibility for ensuring this
cast is valid out of the hands of the compiler (via generics) and specifically
into your hands, hence the warning.

What is the correct way to cast this object back to the
RegistrationInfo Object?

If it's not a RegistrationInfo object there is no way to cast it to one. The
error is caused because the object which chord.retrieve(retrieveKey) returned
was not of class RegistrationInfo but you attempted to cast it into one. You
are not using generics so the compiler can't help. You need to check your code
(the code which you haven't shown us) and see what is being stored and
retrieved.

--
Nigel Wade, System Administrator, Space Plasma Physics Group,
University of Leicester, Leicester, LE1 7RH, UK
E-mail : nmw@ion.le.ac.uk
Phone : +44 (0)116 2523548, Fax : +44 (0)116 2523555