Java Stream API - 计算嵌套列表的项目

Java Stream API - count items of a nested list

提问by dominik

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:

让我们假设我们有一个国家/List<Country>地区列表：每个国家/地区都有对其地区列表的引用：（List<Region>例如美国的州）。像这样的东西：

USA
  Alabama
  Alaska
  Arizona
  ...

Germany
  Baden-Württemberg
  Bavaria
  Brandenburg
  ...

In "plain-old" Java we can count all regions e.g. this way:

在“普通”Java 中，我们可以计算所有区域，例如这样：

List<Country> countries = ...
int regionsCount = 0;

for (Country country : countries) {
    if (country.getRegions() != null) {
        regionsCount += country.getRegions().size();
    }
}

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:

是否可以使用 Java 8 Stream API 实现相同的目标？我想过类似的事情，但我不知道如何使用count()流 API 的方法计算嵌套列表的项目：

countries.stream().filter(country -> country.getRegions() != null).???

回答by fabian

You could use map()to get a Streamof region lists and then mapToIntto get the number of regions for each country. After that use sum()to get the sum of all the values in the IntStream:

您可以使用map()获取Stream区域列表，然后mapToInt获取每个国家/地区的区域数量。之后使用sum()获取 中所有值的总和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();

Note:The benefit of using getRegionsbefore filtering is that you don't need to call getRegionsmore than once.

注意：getRegions在过滤之前使用的好处是您不需要getRegions多次调用。

回答by Vasily Liaskovsky

You may map each country to number of regions and then reduce result using sum:

您可以将每个国家映射到区域数量，然后使用 sum 减少结果：

countries.stream()
  .map(c -> c.getRegions() == null ? 0 : c.getRegions().size())
  .reduce(0, Integer::sum);

回答by Ram Patra

You could even use flatMap()like:

你甚至可以使用flatMap()像：

countries.stream().map(Country::getRegions).flatMap(List::stream).count();

where,

map(Country::getRegions) = returns a Stream<List<Regions>>
flatMap(List::stream) = returns a Stream<Regions>