Java Stream API - 计算嵌套列表的项目 [英] Java Stream API - count items of a nested list
问题描述
我们假设我们有一个国家/地区列表:列表< Country>
,每个国家/地区都有对其区域列表的引用: List< ; Region>
(例如美国的州)。这样的事情:
Let's assume that we have a list of countries: List<Country>
and each country has a reference to a list of its regions: List<Region>
(e.g. states in the case of the USA). Something like this:
USA
Alabama
Alaska
Arizona
...
Germany
Baden-Württemberg
Bavaria
Brandenburg
...
在普通的Java中,我们可以计算所有区域,例如这样:
In "plain-old" Java we can count all regions e.g. this way:
List<Country> countries = ...
int regionsCount = 0;
for (Country country : countries) {
if (country.getRegions() != null) {
regionsCount += country.getRegions().size();
}
}
是否有可能通过Java 8实现相同的目标流API?我想到了类似的东西,但我不知道如何使用 count()
流API方法来计算嵌套列表的项目:
Is it possible to achieve the same goal with Java 8 Stream API? I thought about something similar to this, but I don't know how to count items of nested lists using count()
method of stream API:
countries.stream().filter(country -> country.getRegions() != null).???
推荐答案
你可以使用 map( )
获取 Stream
的区域列表,然后 mapToInt
获取区域数量各国。之后使用 sum()
获取 IntStream
中所有值的总和:
You could use map()
to get a Stream
of region lists and then mapToInt
to get the number of regions for each country. After that use sum()
to get the sum of all the values in the IntStream
:
countries.stream().map(Country::getRegions) // now it's a stream of regions
.filter(rs -> rs != null) // remove regions lists that are null
.mapToInt(List::size) // stream of list sizes
.sum();
注意:使用 getRegions <的好处
在过滤之前,您不需要多次调用 getRegions
。
Note: The benefit of using getRegions
before filtering is that you don't need to call getRegions
more than once.
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