Accessing variables from inner class

Question

I've got some code which defines an anonymous inner class for a callback handler. This handler needs to assign a local variable, see below. I need to assign resp in the callback and refer to it towards the end of the function. I am getting this error in Eclipse however:

The final local variable resp cannot be assigned, since it is defined in an enclosing type

How can I fix this?

DoorResult unlockDoor(final LockableDoor door) {
    final UnlockDoorResponse resp;
    final boolean sent = sendRequest(new UnlockDoorRequest(door), 
       new ResponseAction() {
        public void execute(Session session) 
               throws TimedOutException, RetryException, RecoverException {
            session.watch(UNLOCK_DOOR);
            resp = (UnlockDoorResponse)session.watch(UNLOCK_DOOR);
        }
    });
    DoorResult result;
    if (!sent) {
        return DoorResult.COMMS_ERROR;
    }
    else {
        return DoorResult.valueOf(resp.getResponseCode());
    }
}

Stan · Accepted Answer · 2011-01-25 20:18:05Z

5

Here is a hack that would work in your case:

DoorResult unlockDoor(final LockableDoor door) {
    final UnlockDoorResponse resp[] = { null };
    final boolean sent = sendRequest(new UnlockDoorRequest(door), new ResponseAction() {
        public void execute(Session session)  throws TimedOutException, RetryException, RecoverException {
            session.watch(UNLOCK_DOOR);
            resp[0] = (UnlockDoorResponse)session.watch(UNLOCK_DOOR);
        }
    });
    DoorResult result;
    if (!sent) {
        return DoorResult.COMMS_ERROR;
    }
    else {
        return null == resp[0] ? null : DoorResult.valueOf(resp[0].getResponseCode());
    }
}

If you want a cleaner solution, though, you have to define a named class for your handler, store the response in its field, and retrieve it using an accessor method.

Best regards, Stan.

answered Jan 25, 2011 at 20:18

Stan

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2 Comments

fred basset Over a year ago

Interesting hack; why does the array work but the simple variable not?

Joel Over a year ago

because you are not modifying the final reference that resp[] points too.

jjnguy · Accepted Answer · 2011-01-25 20:14:42Z

3

You could get around this by creating a wrapper class for the response.

class ResponseWrapper {
    UnlockDoorResponse resp;
    void setResponse(UnlockDoorResponse resp) {
        this.resp = resp;
    }
    UnlockDoorResponse getResponse() {
        return resp;
    }
}

Then, your code would look like:

final ResponseWrapper respWrap = new ResponseWrapper();
final boolean sent = sendRequest(new UnlockDoorRequest(door), new ResponseAction() {
    public void execute(Session session)  throws TimedOutException, RetryException, RecoverException {
        session.watch(UNLOCK_DOOR);
        respWrap.setResponse((UnlockDoorResponse)session.watch(UNLOCK_DOOR));
    }
 });
DoorResult result;
if (!sent) {
    return DoorResult.COMMS_ERROR;
}
else {
    return DoorResult.valueOf(respWrap.getResponse().getResponseCode());
}

answered Jan 25, 2011 at 20:14

jjnguy

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1 Comment

Joel Over a year ago

ResponseWrapper needs to be initialised.

Joel · Accepted Answer · 2011-01-26 11:33:33Z

1

Assuming this is your code to change, how about changing sendRequest and ResponseAction.execute to return an instance of UnlockDoorResponse

DoorResult unlockDoor(final LockableDoor door) {
    final UnlockDoorResponse resp = sendRequest(new UnlockDoorRequest(door), new ResponseAction() {
        public UnlockDoorResponse execute(Session session)  throws TimedOutException, RetryException, RecoverException {
            session.watch(UNLOCK_DOOR);
            return (UnlockDoorResponse)session.watch(UNLOCK_DOOR);
        }
    });
    if (resp == null) {
        return DoorResult.COMMS_ERROR;
    }
    else {
        return DoorResult.valueOf(resp.getResponseCode());
    }
}

edited Jan 26, 2011 at 11:33

answered Jan 25, 2011 at 20:17

Joel

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Comments

Robin · Accepted Answer · 2011-01-25 21:05:40Z

If you are going to return results, then use a named inner class instead of an anonymous one. All the other options presented are IMHO ugly hacks (one self admitted ;-)

(OK, @Joel's is not but assumes you can change the interface you are implementing)

Just create an instance of the class with a getter for the result, it is clean and only requires you to implement the single class.

    class MyReponseAction implements ResponseAction {
        private UnlockDoorResponse response; 

        public void execute(Session session)  throws TimedOutException, RetryException, RecoverException {
           session.watch(UNLOCK_DOOR);
           response = (UnlockDoorResponse)session.watch(UNLOCK_DOOR);
        }

        UnlockDoorResponse getResponse() {
            return response;
        }
    }

   DoorResult unlockDoor(final LockableDoor door) {
        ResponseAction action = new MyResponseAction();
        final boolean sent = sendRequest(new UnlockDoorRequest(door), action);

        DoorResult result;
        if (!sent) {
            return DoorResult.COMMS_ERROR;
        }
        else {
            return DoorResult.valueOf(action.getResponse().getResponseCode());
        }
    }

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