分组后流不保留顺序 [英] Stream doesn't preserve the order after grouping

问题描述

我有一个列表名称 availableSeats 我正在按 blockIndex 属性进行排序和分组，如下所示：

I have a List name availableSeats I am sorting and grouping by the blockIndex property like below:

availableSeats.stream()
                .sorted(Comparator.comparing(SeatedTicketAssignment::getBlockIndex))
                .collect(Collectors.groupingBy(SeatedTicketAssignment::getBlockIndex))
                .forEach((block, blockAssignments) -> {
                     //Rest of the code
                } 

问题是分组的结果没有按blockIndex排序。

The problem is that the result of grouping by is not sorted by the blockIndex.

推荐答案

保持记住，收集器#groupingBy（功能）将返回 HashMap ，这不保证订单。如果你想要按顺序排列聚合标识（您的 i％2 == 0 结果）显示，然后您可以使用 LinkedHashMap ：

Keep in mind, Collectors#groupingBy(Function) will return a HashMap, which does not guarantee order. If you want the ordering to be in the order that the aggregate identity (your i % 2 == 0 result) shows up, then you can use LinkedHashMap:

.collect(Collectors.groupingBy(i -> i % 2 == 0, LinkedHashMap::new, Collectors.toList()))

会返回a LinkedHashMap< Boolean，List< SeatedTicketAssignment>> （因为您的收集器按布尔值分组）。此外，由于收集器使用的列表是 ArrayList ，因此它应该保留流相对于列表的迭代顺序。

Would return a LinkedHashMap<Boolean, List<SeatedTicketAssignment>> (since your collector is grouped by a boolean). Additionally, since the list utilized by the collector is an ArrayList, it should preserve the iteration order of the stream relative to the list.

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