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  4. Java 8 Stream and Lambda Expressions – Parsing File Example

Java 8 Stream and Lambda Expressions – Parsing File Example

By 
Eyal Golan user avatar
Eyal Golan
·
Jan. 12, 15 · Interview
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Recently I wanted to extract certain data from an output log.
Here’s part of the log file:

2015-01-06 11:33:03 b.s.d.task [INFO] Emitting: eVentToRequestsBolt __ack_ack [-6722594615019711369 -1335723027906100557]
2015-01-06 11:33:03 c.s.p.d.PackagesProvider [INFO] ===---> Loaded package com.foo.bar
2015-01-06 11:33:04 b.s.d.executor [INFO] Processing received message source: eventToManageBolt:2, stream: __ack_ack, id: {}, [-6722594615019711369 -1335723027906100557]
2015-01-06 11:33:04 c.s.p.d.PackagesProvider [INFO] ===---> Loaded package co.il.boo
2015-01-06 11:33:04 c.s.p.d.PackagesProvider [INFO] ===---> Loaded package dot.org.biz

I decided to do it using the Java8 Stream and Lambda Expression features.

Read the file
First, I needed to read the log file and put the lines in a Stream:

Stream<String> lines = Files.lines(Paths.get(args[1]));

Filter relevant lines
I needed to get the packages names and write them into another file.
Not all lines contained the data I need, hence filter only relevant ones.

lines.filter(line -> line.contains("===---> Loaded package"))

Parsing the relevant lines
Then, I needed to parse the relevant lines.
I did it by first splitting each line to an array of Strings and then taking the last element in that array.
In other words, I did a double mapping. First a line to an array and then an array to a String.

.map(line -> line.split(" "))
.map(arr -> arr[arr.length - 1])

Writing to output file
The last part was taking each string and write it to a file. That was the terminal operation.

.forEach(package -> writeToFile(fw, package));

writeToFile is a method I created.
The reason is that Java File System throws IOException. You can’t use checked exceptions in lambda expressions.

Here’s a full example (note, I don’t check input)

import java.io.FileWriter;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Stream;

public class App {
    public static void main(String[] args) throws IOException {
        Stream<String> lines = null;
        if (args.length == 2) {
            lines = Files.lines(Paths.get(args[1]));
        } else {
            String s1 = "2015-01-06 11:33:03 b.s.d.task [INFO] Emitting: adEventToRequestsBolt __ack_ack [-6722594615019711369 -1335723027906100557]";
            String s2 = "2015-01-06 11:33:03 b.s.d.executor [INFO] Processing received message source: eventToManageBolt:2, stream: __ack_ack, id: {}, [-6722594615019711369 -1335723027906100557]";
            String s3 = "2015-01-06 11:33:04 c.s.p.d.PackagesProvider [INFO] ===---> Loaded package com.foo.bar";
            String s4 = "2015-01-06 11:33:04 c.s.p.d.PackagesProvider [INFO] ===---> Loaded package co.il.boo";
            String s5 = "2015-01-06 11:33:04 c.s.p.d.PackagesProvider [INFO] ===---> Loaded package dot.org.biz";
            List<String> rows = Arrays.asList(s1, s2, s3, s4, s5);
            lines = rows.stream();
        }

        new App().parse(lines, args[0]);

    }

    private void parse(Stream<String> lines, String output) throws IOException {
        final FileWriter fw = new FileWriter(output);

        //@formatter:off
        lines.filter(line -> line.contains("===---> Loaded package"))
        .map(line -> line.split(" "))
        .map(arr -> arr[arr.length - 1])
        .forEach(package -> writeToFile(fw, package));
        //@formatter:on
        fw.close();
        lines.close();
    }

    private void writeToFile(FileWriter fw, String package) {
        try {
            fw.write(String.format("%s%n", package));
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

}



If you enjoyed this article and want to learn more about Java Streams, check out this collection of tutorials and articles on all things Java Streams.

File system Stream (computing) Java (programming language)

Published at DZone with permission of Eyal Golan, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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  • Using Java Stream Gatherers To Improve Stateful Operations
  • Thread-Safety Pitfalls in XML Processing
  • Java Stream API: 3 Things Every Developer Should Know About

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