I'm not sure you could implement a useful flatMap() for Collector-like types, at least without forcing the collector to hold all the received values in memory,

I'm not sure you could implement a useful flatMap() for Collector-like types, at least without forcing the collector to hold all the received values in memory, which would defeat the purpose. It would be like a function that takes - a Collector that produces an X - a function that takes an X and returns a Collector that produces an Y and returns a Collector that produces an Y. The thing is: while being fed, the result Collector should first feed the initial Collector and, at some time, "switch" to the Collector produced by the function. But when to perform the switch?