Primary keys in entity beans

Question

I am creating an assignment for an EJB class that I am teaching. The assignment will ask students to create an entity bean to represent messages posted to an internet bulletin board. The fields in the entity bean are ID number, topic, user name, date, and the message. This bean will use container-managed persistence.
The ID number of each message will be used for the primary key.
The client will present the user with a simple menu (at the DOS prompt) with options to list all messages, search for messages, add messages, etc.
When a new message is added, I don't want the user to have to enter a unique ID - it should be generated in the code somewhere.
What I'm looking for is the absolute simplest way to get an ID for each new message. (I'm planning a series of assignments, each getting more complex, and this is the simplest one).
I was thinking of having the client get a list of all the messages, make a list of all the IDs used, find the largest ID, and add 1.
Can anyone suggest any other techniques? Thanks!
-J

Answer 1

Two approaches:
you could initialize the db with one row and use some (any) number as the primary key
then when adding a new row, access the last row in the db to get it's primary key and add 1 to it.
You could also use date+time as a string, and assuming no one adds messages at the exact date+time, then it will work.
To work around the problem of having two messages with identical date+time (assuming lots of messages coming through), you can catch the DB exception and resubmit the entry with an updated date+time. With very heavy traffic this will eventually fail. But for what you are doing it should work fine.
Finally, you should not run into the identical problem unless you are using asynchronous message beans. I doubt you are, so the EJB container will handle it I suspect.