Java 8 stream Map to List

问题:

Given that I have some function that takes two parameters and returns one value , is it possible to convert a Map to a List in a Stream as a non-terminal operation? The nearest I cam find is to use forEach on the map to create instances and add them to a pre-defined List, then start a new Stream from that List. Or did I just miss something?

Eg: The classic "find the 3 most frequently occurring words in some long list of words"

wordList.stream().collect(groupingBy(Function.identity, Collectors.counting))).

(now I want to stream the entrySetof that map)

sorted((a,b) -> a.getValue().compareTo(b.getValue))).limit(3).forEach(print...

回答1:

You should get the entrySet of the map and glue the entries to the calls of your binary function:

inputMap.entrySet().stream().map(e->myFun(e.getKey(),e.getValue()));

The result of the above is a stream of T instances.

Update

Your additional example confirms what was discussed in the comments below: group by and sort are by their nature terminal operations. They must be performed in full to be able to produce even the first element of the output, so involving them as non-terminal operations doesn't buy anything in terms of performance/memory footprint.

It happens that Java 8 defines sorted as a non-terminal operation, however that decision could lead to deceptive code because the operation will block until it has received all upstream elements, and will have to retain them all while receiving.