收集HashSet / Java 8 / Regex Pattern / Stream API

问题描述

最近我改变版本的JDK 8而不是我的项目7，现在我使用Java 8的新功能覆盖一些代码片段。

Recently I change version of the JDK 8 instead 7 of my project and now I overwrite some code snippets using new features that came with Java 8.

final Matcher mtr = Pattern.compile(regex).matcher(input);

HashSet<String> set = new HashSet<String>() {{
    while (mtr.find()) add(mtr.group().toLowerCase());
}};

如何使用Stream API编写此代码？

推荐答案

如果您重复使用JDK的分割器实现，可以非常简单地实现一个 Matcher 提供 Spliterators.AbstractSpliterator ：

A Matcher-based spliterator implementation can be quite simple if you reuse the JDK-provided Spliterators.AbstractSpliterator:

public class MatcherSpliterator extends AbstractSpliterator<String[]>
{
  private final Matcher m;

  public MatcherSpliterator(Matcher m) {
    super(Long.MAX_VALUE, ORDERED | NONNULL | IMMUTABLE);
    this.m = m;
  }

  @Override public boolean tryAdvance(Consumer<? super String[]> action) {
    if (!m.find()) return false;
    final String[] groups = new String[m.groupCount()+1];
    for (int i = 0; i <= m.groupCount(); i++) groups[i] = m.group(i);
    action.accept(groups);
    return true;
  }
}

请注意，spliterator提供匹配器组，而不仅仅是完全匹配。另请注意，此分割器支持并行性，因为 AbstractSpliterator 实现分割策略。

Note that the spliterator provides all matcher groups, not just the full match. Also note that this spliterator supports parallelism because AbstractSpliterator implements a splitting policy.

通常， ：

public static Stream<String[]> matcherStream(Matcher m) {
  return StreamSupport.stream(new MatcherSpliterator(m), false);
}

这为你提供了一个强大的基础来简洁地编写各种复杂的正则表达式逻辑，例如：

This gives you a powerful basis to concisely write all kinds of complex regex-oriented logic, for example:

private static final Pattern emailRegex = Pattern.compile("([^,]+?)@([^,]+)");
public static void main(String[] args) {
  final String emails = "[e-mail], [e-mail], [e-mail]";
  System.out.println("User has e-mail accounts on these domains: " +
      matcherStream(emailRegex.matcher(emails))
      .map(gs->gs[2])
      .collect(joining(", ")));
}

哪些打印

User has e-mail accounts on these domains: gmail.com, yahoo.com, tijuana.com

为了完整性，您的代码将被重写为

For completeness, your code will be rewritten as

Set<String> set = matcherStream(mtr).map(gs->gs[0].toLowerCase()).collect(toSet());

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