Match a pattern and write the stream to a file using Java 8 Stream

Question

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();

        }



    }

}

Please help me. Thanks!

EDIT: Answers to the questions..

No there are no multiple quoted texts.

I could have used simple loop. But I want to use Java 8 streams

Answer 1

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

Note: It has been added in Java 9 as Matcher.results().

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();
    }
}

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);
    }
}

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?

Match a pattern and write the stream to a file using Java 8 Stream

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