匹配模式并使用Java 8 Stream将流写入文件 [英] Match a pattern and write the stream to a file using Java 8 Stream

问题描述

我正在尝试读取一个大文件，并将引号"中的文本提取出来，并将这些行放入集合中，然后使用Java 8 Stream 将集合的内容写入文件中./p>

I'm trying to read a huge file and extract the text within "quotes" and put the lines into a set and write the content of the set to a file using Java 8 Stream.

public class DataMiner {

    private static final Pattern quoteRegex = Pattern.compile("\"([^\"]*)\"");

    public static void main(String[] args) {

        String fileName = "c://exec.log";
        try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
            Set<String> dataSet = stream.
                    //How do I Perform pattern match here
                    .collect(Collectors.toSet());
            Files.write(Paths.get(fileName), dataSet);

        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

请帮助我.谢谢！

问题的答案.

1. 不，没有多引号的文字.
2. 我本可以使用简单的循环.但是我想使用Java 8流

推荐答案

不幸的是，Java正则表达式类没有提供匹配结果的流，只有

Unfortunately, the Java regular expression classes don't provide a stream for matched results, only a splitAsStream() method, but you don't want split.

注意:已在Java 9中添加为 Matcher.results().

不过，您可以自己为其创建通用帮助程序类:

You can however create a generic helper class for it yourself:

public final class PatternStreamer {
    private final Pattern pattern;
    public PatternStreamer(String regex) {
        this.pattern = Pattern.compile(regex);
    }
    public Stream<MatchResult> results(CharSequence input) {
        List<MatchResult> list = new ArrayList<>();
        for (Matcher m = this.pattern.matcher(input); m.find(); )
            list.add(m.toMatchResult());
        return list.stream();
    }
}

然后，通过使用

Then your code becomes easy by using flatMap():

private static final PatternStreamer quoteRegex = new PatternStreamer("\"([^\"]*)\"");
public static void main(String[] args) throws Exception {
    String inFileName = "c:\\exec.log";
    String outFileName = "c:\\exec_quoted.txt";
    try (Stream<String> stream = Files.lines(Paths.get(inFileName))) {
        Set<String> dataSet = stream.flatMap(quoteRegex::results)
                                    .map(r -> r.group(1))
                                    .collect(Collectors.toSet());
        Files.write(Paths.get(outFileName), dataSet);
    }
}

由于您一次只能处理一行，因此临时 List 很好.如果输入字符串很长并且匹配很多，那么 Spliterator 是一个更好的选择.请参见如何创建正则表达式匹配流?

Since you only process a line at a time, the temporary List is fine. If the input string is very long and will have a lot of matches, then a Spliterator would be a better choice. See How do I create a Stream of regex matches?

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