Trouble with references....

Question

I am trying to write and test a simple syncronized method that 'locks' a record in a database. The reason it is not working is because the instance of Record does not seem to be retaining the value I am setting/mutating. I have been struggling wiht the code for a few days now and I think I found the problem code.
public synchronized long lock(int recNo) throws RecordNotFoundException {
     long lockCookie = -1;
     if(recNo >= 0 && recNo <= dbData.size()) {
          //get the record
          Record r = (Record)dbData.get(recNo);
          System.out.println("Extracted Instance Lock Cookie " + r.getLockCookie() + " Rec No " + r.getRecordNumber());
           If the record is locked then wait for it to unlock
          while (r.isLocked()) {
               try {
                    System.out.println("Waiting");
                    wait();
               } catch (InterruptedException ie) {}
          /* create a cookie based on the current time in milliseconds */
          lockCookie = new Date().getTime();
          /* set the lock cookie to the record*/
          r.setLockCookie(lockCookie);
          /* put the updated instance back into the ArrayList */
          dbData.set(recNo,r);
     } else {
          throw new RecordNotFoundException("Cannot Lock - Record Not Found");
     return lockCookie;
}As you can see I create a 'lock cookie' and the call Record.setLockCookie(long). I assumed that this would change the reference that i get() from the ArrayList dbData. But if I try to lock that record again the SOP prints "Extracted Lock Cookie: -1" which is the default value when it should be the value I set. I added the line dbData.set(recNo,r) to ensure that the Object that has been modified was put back in the ArrayList. But the SOP still gives -1 as the value for the lockCookie. My app has a client that requests a lock, when it has a lock it can modify a record, but now if another client requests a lock on the same record it also recieves a lock becuase the isLocked() method is always returning false bacuase the records lockCookie after I extract it from the ArrayList is -1. I can't see anything wrong with my code so I'm hoping someone else can.

Answer 1

As best as I can tell your question is the following.
- You have a class of type 'Record'
- Using an int index you set an instance into an array list.
- You set an attribute of that instance.
- When you retrieve it using the index the attribute is not set.
Possible reasons.
- You are not using the same ArrayList
- When you retrieve it you are not retreiving it from the same source
- Something is overwriting it.
- The set method doesn't work.
- The get method doesn't work.
Depending on what you are doing it could also be because
- You are running two different versions of the VM (and the data is not persisted and sync between.)
- You are not persisting the data.