I have a

Map<FileKey, Deferred<String>>

where I want to cache (expensive-to-compute) SHA-512 results. My objective is to only calculate hash values once, on the first retrieval from the Map. So I know I want some kind of Future object. I know I can start the computation in the getter (unless it's already complete) and then just return the result normally. But is Deferred the best way to do this or is there a different construct/pattern I should be looking at instead? I probably want to block on the retrieval, so Deferred may not be the best choice.

Thanks. So the original idea is feasible. Will be worth doing for web requests but probably not local files.