[英]Java reduce a collection of string to a map of occurence
Consider the a list as id1_f, id2_d, id3_f, id1_g
, how can I use stream to get a reduced map in format of <String, Integer>
of statistics like: 将列表视为id1_f, id2_d, id3_f, id1_g
,如何使用流来获取统计信息的<String, Integer>
格式的简化映射,如:
id1 2
id2 1
id3 1
Note: the key is part before _
. 注意:键在_
之前是部分。 Is reduce
function can help here? reduce
功能可以帮助吗?
This will get the job done: 这将完成工作:
Map<String, Long> map = Stream.of("id1_f", "id2_d", "id3_f", "id1_g")
.collect(
Collectors.groupingBy(v -> v.split("_")[0],
Collectors.counting())
);
You can also use the toMap
collector: 您还可以使用toMap
收集器:
myList.stream()
.collect(Collectors.toMap((String s) -> s.split("_")[0],
(String s) -> 1, Math::addExact);
if you care about the order of the elements then dump the result into a LinkedHashMap
. 如果您关心元素的顺序,则将结果转储到LinkedHashMap
。
myList.stream()
.collect(Collectors.toMap((String s) -> s.split("_")[0],
(String s) -> 1, Math::addExact,
LinkedHashMap::new));
A non-stream approach using Map::merge : 使用Map :: merge的非流方法:
Map<String, Integer> result = new LinkedHashMap<>();
myList.forEach(s -> result.merge(s.split("_")[0], 1, Math::addExact));
Since you want to count the elements, I'd suggest using Guava 's Multiset
interface, which is dedicated to such purpose. 由于你想要计算元素,我建议使用Guava的Multiset
接口,它专门用于此目的。
The definition of Multiset
from its JavaDoc: 从JavaDoc中定义Multiset
:
A collection that supports order-independent equality, like
Set
, but may have duplicate elements. 支持与顺序无关的相等的集合,如Set
,但可能具有重复的元素。 A multiset is also sometimes called a bag . multiset有时也被称为包 。Elements of a multiset that are equal to one another are referred to as occurrences of the same single element. 多重集是彼此相等的元件被称为相同的单个元件的出现 。 The total number of occurrences of an element in a multiset is called the count of that element. 多集中元素的出现总数称为该元素的计数 。
Here are two ways to use it: 以下是两种使用方法:
1) Without the Stream API: 1)没有Stream API:
ImmutableMultiset<String> multiset2 = ImmutableMultiset.copyOf(Lists.transform(
list, str -> StringUtils.substringBefore(str, "_")
));
2) Using the Stream API: 2)使用Stream API:
ImmutableMultiset<String> multiset = list.stream()
.map(str -> StringUtils.substringBefore(str, "_"))
.collect(ImmutableMultiset.toImmutableMultiset());
Note that instead of using something like s.split("_")[0]
, I used Apache Commons Lang 's StringUtils.substringBefore
, which I find much more readable. 请注意,我没有使用类似s.split("_")[0]
,而是使用了Apache Commons Lang的StringUtils.substringBefore
,我发现它更具可读性。
You retrieve the counts of the elements using Multiset.count()
method. 您可以使用Multiset.count()
方法检索元素的计数。
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