在Java 8中限制groupBy

Question

如何通过每个条目限制groupBy？

例如(基于此示例: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？让我们假设我只想要每个老师的前N个课程.怎么做到呢？

Answer 1

可以引入一个新的收集器来限制结果列表中的元素数量.

此收集器将保留列表的头元素(按遭遇顺序).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.

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)
                     ));

Answer 2

您可以使用collectionAndThen在结果列表上定义整理器操作。这样，您可以限制，过滤，排序，...列表：

int limit = 2;

Map<String, List<StudentClass>> groupByTeachers =
    studentClasses.stream()
                  .collect(
                       groupingBy(
                           StudentClass::getTeacher,
                           collectingAndThen(
                               toList(),
                               l -> l.stream().limit(limit).collect(toList()))));