在Java 8中限制groupBy [英] Limit groupBy in Java 8

问题描述

如何通过每个条目限制groupBy？

How can I limit groupBy by each entry?

例如（基于此示例： stream groupBy ）：

For example (based on this example: stream groupBy):

studentClasses.add(new StudentClass("Kumar", 101, "Intro to Web"));
studentClasses.add(new StudentClass("White", 102, "Advanced Java"));
studentClasses.add(new StudentClass("Kumar", 101, "Intro to Cobol"));
studentClasses.add(new StudentClass("White", 101, "Intro to Web"));
studentClasses.add(new StudentClass("White", 102, "Advanced Web"));
studentClasses.add(new StudentClass("Sargent", 106, "Advanced Web"));
studentClasses.add(new StudentClass("Sargent", 103, "Advanced Web"));
studentClasses.add(new StudentClass("Sargent", 104, "Advanced Web"));
studentClasses.add(new StudentClass("Sargent", 105, "Advanced Web"));

此方法返回简单组：

   Map<String, List<StudentClass>> groupByTeachers = studentClasses
            .stream().collect(
                    Collectors.groupingBy(StudentClass::getTeacher));

如果我想限制返回的集合怎么办？
我们假设我只想要每个老师的前N个课程。如何做？

What if I want to limit the returned collections? Let's assume I want only the first N classes for every teacher. How can it be done?

推荐答案

有可能引入一个新的收集器来限制结果列表中的元素数量。

It would be possible to introduce a new collector that limits the number of elements in the resulting list.

此收藏家将保留列表的头部元素（按顺序排列）。在收集期间达到限制时，累加器和组合器会丢弃每个元素。组合器代码有点棘手，但这样做的好处是不会添加额外的元素，只是为了以后丢弃。

This collector will retain the head elements of the list (in encounter order). The accumulator and combiner throw away every elements when the limit has been reached during collection. The combiner code is a little tricky but this has the advantage that no additional elements are added only to be thrown away later.

private static <T> Collector<T, ?, List<T>> limitingList(int limit) {
    return Collector.of(
                ArrayList::new, 
                (l, e) -> { if (l.size() < limit) l.add(e); }, 
                (l1, l2) -> {
                    l1.addAll(l2.subList(0, Math.min(l2.size(), Math.max(0, limit - l1.size()))));
                    return l1;
                }
           );
}

然后像这样使用它：

Map<String, List<StudentClass>> groupByTeachers = 
       studentClasses.stream()
                     .collect(groupingBy(
                          StudentClass::getTeacher,
                          limitingList(2)
                     ));

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