Data Queue/Pool Design

I am trying to writing a data-retrieval program for my data server. In my design, the server will keep on generating data continuously. Now I want that whenever client side user connects to server, if the server has any already generated data, it will get ALL available data back; and if user has not connected to server to get data, all the generated data will be kept in a Data Queue/Pool and waiting for user to retrieve them at a later time. - I think this is very similar to mail program.
My question is how to design such a Data Queue/Pool? There are two main problem here: 1). Just like mail server, different user may have different data, so each user must have a data pool ( for him/her to retrieve), so how to keep data for different user?
2). When user is retrieving data, at the same time, the data may be adding to the pool, so how to prevent data lost?
Any one can show me some code fragment of such Queue/pool?
Many thanks!

If a got you right (i may repeat you anyway) you have:
1. A set of users
2. For every user there is some data (that is in genereal collected)
3. When user connects all available data is send to him and removed from the server.
If this is the case than:
1. Write some queue (lets name it Que) that holds a data for a given user. This queue has two main mathods:
- putData - that you use to store data in queue
- getAllData - that removes all data from the queue and returns it to the caller...
These two methods are synchronized... (probably you will use some collection class, like List or Set, in this Que to keep data)...
When server ganarates some data use putData method... When user connects use getAllData method... this is of course obvious to you :)))
2. You must have some kind of user management... maybe you will use Map to retrieve Que for given user name - you can make this as you wish...
There is one another thing that came to my mind while i was reading your post... What will happen if server generates a lot of data... Than for you good you should use hdd to keep data and may by some kind of object pools... but that is not important for the moment... :)))

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