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SessionListener online users - Splitting up lists

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I'm using a SessionListener to check if a user is online or not.
It basically keeps a List of all online users.
Too check if one is online or not I use "contains()".

If there were say a few 1000 users online it would take a while to perform "contains()".
That's why I thought of splitting up the lists in a few lists identified by the first 3 chars or so of the users id.

Is this the way to go, or are there other, better ways of doing this?

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The obvious choice for a collection is to use a implementation of the Set interface, for example HashSet - essentially a Map where the entry (user name) is both value and key.

This sort of lookup uses a Hashcode and is very very fast compared to brute force lookups with (for example) an ArrayList.


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