CareerCup

Keep a linked list of <Session, timestamp> pairs.
Keep a hash table of Session-> SessionTimestamp list node, for constant time lookup into the list above.
Every time a Session is used, find the list node, update the timestamp, and move the node to the end of the list. This means the list goes from last active, to latest active Sessions.

Every once in a while (SessionManager class can be an active object), go through the list and purge sessions that haven't been active in N seconds. Note that when you're going through the list purging Sessions, you can stop the search as soon as you reach a Session that has been active in the last N sessions, since the list is sorted.

I will create threads. Will count the number of sessions at that time
Let say there are N sessions. Will create N/n threads let say every thread scans n sessions.
Now sessions can be removed based on their time stamp

I'd like to use Hashmap with doubly linked list

Use a max heap - arrange it using the 'last used' timestamp of the session objects. When a session is used, update its timestamp, remove it from the heap and heapify. Then place it back in the heap with the new timestamp.

Identifying the oldest sessions is Ologn.

i will use treemap with session as key and lastused time as value.
will provide a comparator to sort according to value in ascending order.
will purge all entries after lastused value 30 secs.