如何将多个谓词应用于java.util.Stream？ [英] How to apply multiple predicates to a java.util.Stream?

问题描述

如何将多个谓词应用于 java.util.Stream的 filter（）方法？

How can I apply multiple predicates to a java.util.Stream's filter() method?

这就是我现在所做的，但我并不喜欢它。我有一个集合的东西，我需要根据过滤器（谓词）的集合来减少事物的数量：

This is what I do now, but I don't really like it. I have a Collection of things and I need to reduce the number of things based on the Collection of filters (predicates):

Collection<Thing> things = someGenerator.someMethod();
List<Thing> filtered = things.parallelStream().filter(p -> {
   for (Filter f : filtersCollection) {
      if (f.test(p))
        return true;
   }
   return false;
}).collect(Collectors.toList());

我知道如果我事先了解过滤器的数量，我可以这样做：

I know that if I knew number of filters up-front, I could do something like this:

List<Thing> filtered = things.parallelStream().filter(filter1).or(filter2).or(filter3)).collect(Collectors.toList());

但是如何在不混合编程风格的情况下应用未知数量的谓词？要知道它看起来有点丑......

But how can I apply unknown number of predicates without mixing programming styles? For know it looks sort of ugly...

推荐答案

我假设您的过滤器是一种与 java.util.function.Predicate 不同的类型，这意味着它需要适应它。一种可行的方法如下：

I am assuming your Filter is a type distinct from java.util.function.Predicate, which means it needs to be adapted to it. One approach which will work goes like this:

things.stream().filter(t -> filtersCollection.stream().anyMatch(f -> f.test(t)));

这会为每个谓词评估重新创建过滤器流带来轻微的性能损失。为了避免这种情况，你可以将每个过滤器包装成谓词并组成它们：

This incurs a slight performance hit of recreating the filter stream for each predicate evaluation. To avoid that you could wrap each filter into a Predicate and compose them:

things.stream().filter(filtersCollection.stream().<Predicate>map(f -> f::test)
                       .reduce(Predicate::or).orElse(t->false));

但是，现在每个过滤器都在它自己的 Predicate ，引入一个更多级别的间接，不明确哪种方法会有更好的整体表现。

However, since now each filter is behind its own Predicate, introducing one more level of indirection, it is not clear-cut which approach would have better overall performance.

没有适应性考虑（如果你的过滤器碰巧是谓词）问题陈述变得简单得多，第二种方法明显胜出：

Without the adapting concern (if your Filter happens to be a Predicate) the problem statement becomes much simpler and the second approach clearly wins out:

things.stream().filter(
   filtersCollection.stream().reduce(Predicate::or).orElse(t->true)
);

这篇关于如何将多个谓词应用于java.util.Stream？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！