Java 8 流映射<K,V>列出<T> [英] Java 8 stream Map<K,V> to List<T>

问题描述

鉴于我有一些函数接受两个参数并返回一个值，是否可以将 Map 转换为 Stream 中的 List 作为非终端操作?我找到的最近的方法是在地图上使用 forEach 创建实例并将它们添加到预定义的列表中，然后从该列表开始一个新的流.还是我只是错过了什么?

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?

例如:经典的在一些长单词列表中找到 3 个最常出现的单词"

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

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

(现在我想流式传输该地图的 entrySet)

(now I want to stream the entrySetof that map)

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

推荐答案

您应该获得映射的 entrySet 并将条目粘合到二进制函数的调用中:

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()));

上面的结果是一个T实例的流.

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

您的附加示例证实了以下评论中所讨论的内容:group by 和 sort 本质上是终端操作.它们必须完整地执行才能产生输出的第一个元素，因此将它们作为非终端操作使用不会在性能/内存占用方面带来任何好处.

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.

Java 8 恰好将 sorted 定义为非终端操作，但是该决定可能导致欺骗性代码，因为该操作将阻塞，直到它收到所有上游元素，并且必须保留

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.

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