What is invokeAll versus invokeAny, and where does CompletionService fit?

invokeAll submits a batch and blocks until every task is done (or the timeout expires), returning futures in the order of the input, all of them already complete. invokeAny is the racing variant: the first task to return normally wins, its value is returned, and the others are cancelled.

Neither helps when you want to process results in completion order rather than submission order, and writing that with Future.get() in a loop wastes time on a slow first element. ExecutorCompletionService solves it: it pushes each finished task onto a queue you take() from, so you always handle whatever is ready.

In modern code CompletableFuture.allOf / anyOf covers the same ground non-blockingly, and StructuredTaskScope covers it with cancellation built in.