如何从Java的自定义谓词列表中创建谓词? [英] How to make a Predicate from a custom list of Predicates in Java?

问题描述

我对编程还比较陌生，在过去的两天里，我一直想知道如何制作一个由其他谓词的自定义列表构成的谓词.所以我想出了一种解决方案.下面是一个代码片段，应该可以给您一个想法.因为我是基于仅阅读各种文档而编写的，所以我有两个问题:1/这是一个好的解决方案吗?2/还有其他建议的解决方案吗?

I'm relatively new to programming and I have been wondering for past two days how to make a Predicate that is made from a custom list of other Predicates. So I've came up with some kind of solution. Below is a code snippet that should give you an idea. Because I have written it based on solely reading various pieces of documentations I have two questions: 1/ is it a good solution? 2/ is there some other, recommended solution for this problem?

public class Tester {
  private static ArrayList<Predicate<String>> testerList;

  //some Predicates of type String here...

  public static void addPredicate(Predicate<String> newPredicate) {
    if (testerList == null) 
                 {testerList = new ArrayList<Predicate<String>>();}
    testerList.add(newPredicate);
  }

  public static Predicate<String> customTesters () {
    return s -> testerList.stream().allMatch(t -> t.test(s));

  }
}

推荐答案

您可以有一个静态方法，该方法可以接收许多谓词并返回所需的谓词:

You could have a static method that receives many predicates and returns the predicate you want:

public static <T> Predicate<T> and(Predicate<T>... predicates) {
    // TODO Handle case when argument is null or empty or has only one element
    return s -> Arrays.stream(predicates).allMatch(t -> t.test(s));
}

一个变体:

public static <T> Predicate<T> and(Predicate<T>... predicates) {
    // TODO Handle case when argument is null or empty or has only one element
    return Arrays.stream(predicates).reduce(t -> true, Predicate::and);
}

这里我正在使用

Here I'm using Stream.reduce, which takes the identity and an operator as arguments. Stream.reduce applies the Predicate::and operator to all elements of the stream to produce a result predicate, and uses the identity to operate on the first element of the stream. This is why I have used t -> true as the identity, otherwise the result predicate might end up evaluating to false.

用法:

Predicate<String> predicate = and(s -> s.startsWith("a"), s -> s.length() > 4);

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