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> With a concurrent hash table, you have to hold a lock on the whole datastructure

I would suspect that the entire point of a concurrent hash table is to avoid exactly that. Not much point otherwise.

Not if you don't want the hash table to change while doing some read only processing on it. Immutability ensures that the reference you get always point towards a object that is in a consistent state.

That is not the point. The point is that the hash table can be updated in a way (e.g., atomic) that is invisible to other readers and thefore they always see a consistent version of the hash table itself.

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