如何提取List< D>来自HashMap< E，R>使用流 [英] How to extract a List<D> from a HashMap<E,R> using stream

问题描述

考虑到这些限制，我想知道如何从HashMap<E,R>中提取List<D>:

I want to know how to extract a List<D> from a HashMap<E,R> considering these constraints:

• E是一个自定义类；
• R是一个包含Set<D>自定义对象的自定义类；

• E is a custom class;
• R is a custom class containing a Set<D> of custom objects;

我尝试过的方法:我尝试在中解决此问题这个问题.

What I have tried: I tried addressing the issue in this question.

在以前的情况下，我有一个简单的Map<E,List<R>>，但是在这种情况下，我必须访问具有目标Set<D>的R类.

In that previous case, I had a simple Map<E,List<R>>, but in this case I have to access the R class which has the targeted Set<D>.

在代码的以下部分中，我要做的是获取国家名称等于给定参数的Set<D>的元素.

What I want to do in the following portion of the code is to get the Elements of the Set<D> whose country names are equal to the given parameter.

我尝试使用相同的解决方案:

I have tried using the same solution :

Map<E,R> map = new HashMap<E,R>();
public List<D> method(String countryname) { 
   return map.values().stream().filter((x)->{
        return x.getSet().stream().anyMatch((t) -> {
            return t.getCountry().equals(countryname); 
        });
    })
    .map(R::getSet)
    .flatMap(List::stream)
    .collect(Collectors.toList()); //does not compile
}

// the R class
class R {
    private Set<D> set = new TreeSet<D>();
    //getters & setters & other attributes
}

推荐答案

我相信flatMap步骤是错误的，因为您的map步骤将您的Stream<R>转换为Stream<Set<D>>，因此flatMap(List::stream)应该是flatMap(Set::stream):

I believe the flatMap step is wrong, since your map step transforms your Stream<R> to a Stream<Set<D>>, so flatMap(List::stream) should be flatMap(Set::stream):

return map.values()
          .stream()
          .filter(x -> x.getSet().stream().anyMatch(t -> t.getCountry().equals(countryname)))
          .map(R::getSet)
          .flatMap(Set::stream)
          .collect(Collectors.toList());

此外，正如您在上面注意到的那样，如果不必使用花括号，则代码可以更具可读性.

Besides, as you can notice above, the code can be much more readable if you avoid using curly braces when you don't have to.

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