如何使用 Java8lambda 从流中获取一系列项目？

在之前的问题[ 如何在 Java8中动态地进行过滤？]中，Stuart Marks 给出了一个很好的答案，并提供了几个有用的实用程序来处理从流中选择 topN 和 topPercent 的问题。

我会在这里列出他最初的回答:

@FunctionalInterface
public interface Criterion {
Stream<Widget> apply(Stream<Widget> s);
}


Criterion topN(Comparator<Widget> cmp, long n) {
return stream -> stream.sorted(cmp).limit(n);
}


Criterion topPercent(Comparator<Widget> cmp, double pct) {
return stream -> {
List<Widget> temp =
stream.sorted(cmp).collect(toList());
return temp.stream()
.limit((long)(temp.size() * pct));
};
}

我的问题是:

[1] How to get top items from 3 to 7 from a stream with certain amount of items, so if the stream has items from A1, A2 .. A10, the call to

topNFromRange(Comparator<Widget> cmp, long from, long to) = topNFromRange(comparing(Widget::length), 3L, 7L)

将返回{ A3，A4，A5，A6，A7}

我能想到的最简单的方法是从原始文件中获取 top 7[ T7] ，从原始文件中获取 top 3[ T3] ，然后获取 T7-T3。

[2] How to get top items from top 10% to top 30% from a stream with certain amount of items, so if the stream has items from X1, X2 .. X100, the call to

topPercentFromRange(Comparator<Widget> cmp, double from, double to) = topNFromRange(comparing(Widget::length), 0.10, 0.30)

将返回{ X10，X11，X12，... ，X29，X30}

我能想到的最简单的方法是从原件中取出最上面的30% [ TP30] ，从原件中取出最上面的10% [ TP10] ，然后得到 TP30-TP10。

有什么更好的方法可以使用 Java8Lambda 简洁地表达上述情况？

120904

小开

To get a range from a Stream<T>, you can use skip(long n) to first skip a set number of elements, and then you can call limit(long n) to only take a specific amount of items.

Consider a stream with 10 elements, then to get elements 3 to 7, you would normally call from a List:

list.subList(3, 7);

Now with a Stream, you need to first skip 3 items, and then take 7 - 3 = 4 items, so it becomes:

stream.skip(3).limit(4);

As a variant to @StuartMarks' solution to the second answer, I'll offer you the following solution which leaves the possibility to chain intact, it works similar to how @StuartMarks does it:

private <T> Collector<T, ?, Stream<T>> topPercentFromRangeCollector(Comparator<T> comparator, double from, double to) {
return Collectors.collectingAndThen(
Collectors.toList(),
list -> list.stream()
.sorted(comparator)
.skip((long)(list.size() * from))
.limit((long)(list.size() * (to - from)))
);
}

and

IntStream.range(0, 100)
.boxed()
.collect(topPercentFromRangeCollector(Comparator.comparingInt(i -> i), 0.1d, 0.3d))
.forEach(System.out::println);

This will print the elements 10 through 29.

It works by using a Collector<T, ?, Stream<T>> that takes in your elements from the stream, transforms them into a List<T>, then obtains a Stream<T>, sorts it and applies the (correct) bounds to it.

Criterion topPercentFromRange(Comparator<Widget> cmp, double from, double to) { return stream -> { List<Widget> temp = stream.sorted(cmp).collect(toList()); return temp.stream() .skip((long)(temp.size() * from)) .limit((long)(temp.size() * (to - from))); }; }