如何使用java8按谓词对列表进行分区？ [英] How to partition a list by predicate using java8?

问题描述

我有一个 a 列表，我想将其拆分为几个小清单。

I have a list a which i want to split to few small lists.

说出所有项目包含aaa的所有内容，包含bbb和更多谓词。

say all the items that contains with "aaa", all that contains with "bbb" and some more predicates.

如何使用java8执行此操作？

How can I do so using java8?

我看到这个发布但它只分成了2个列表。

I saw this post but it only splits to 2 lists.

public void partition_list_java8() {

    Predicate<String> startWithS = p -> p.toLowerCase().startsWith("s");

    Map<Boolean, List<String>> decisionsByS = playerDecisions.stream()
            .collect(Collectors.partitioningBy(startWithS));

    logger.info(decisionsByS);

    assertTrue(decisionsByS.get(Boolean.TRUE).size() == 3);
}

我看到了这个 post ，但它在Java 8之前很老了。

I saw this post, but it was very old, before java 8.

推荐答案

就像在 @RealSkeptic 中解释的一样a href =https://stackoverflow.com/questions/36678571/how-to-partition-a-list-by-predicate-using-java8/36678648#comment60947120_36678571>评论 谓词只能返回两个结果：true和false。这意味着您只能将数据拆分为两组。

您需要的是某种函数，这将允许您确定一些应该组合在一起的元素的共同结果。在你的情况下，这样的结果可能是小写的第一个字符（假设所有字符串都不是空的 - 至少有一个字符）。

Like it was explained in @RealSkeptic comment Predicate can return only two results: true and false. This means you would be able to split your data only in two groups.
What you need is some kind of Function which will allow you to determine some common result for elements which should be grouped together. In your case such result could be first character in its lowercase (assuming that all strings are not empty - have at least one character).

现在 Collectors.groupingBy（function） 您可以将所有元素分组到单独的列表中，并将它们存储在Map中，其中key将是用于分组的常用结果（如第一个字符）。

Now with Collectors.groupingBy(function) you can group all elements in separate Lists and store them in Map where key will be common result used for grouping (like first character).

所以你的代码看起来像

Function<String, Character> firstChar =  s -> Character.toLowerCase(s.charAt(0));

List<String> a = Arrays.asList("foo", "Abc", "bar", "baz", "aBc");
Map<Character, List<String>> collect = a.stream()
        .collect(Collectors.groupingBy(firstChar));

System.out.println(collect);

输出：

{a=[Abc, aBc], b=[bar, baz], f=[foo]}

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