Question

I’m begginer and keep yourself in hends.

I need to do organize multithreadings find in files:

User input where find(path) and what find(word);

• First thread finds .txt files in folder and add result to queue;
• When queue has some file => Second thread start find in this file
  what need to find(word).
• If was finded success would show path this
  file + how offen times this word meets in file.

Qestions:

• Can we use ArrayList (or exist any alternatives) for queue which works with few threads?
• How to do if queue is empty, Second thread don’t start but waits when First finded need file?
• Need we use synchronized for this task and inherited MultiThreadingSearch(or better to use composition)?

Code:

import java.util.*;
import java.io.*;

class ArrayListOfFiles {
    private Node first, last;

    private class Node {
        String item;
        Node next;
    }

    public boolean isEmpty() {
        return first == null;
    }

    public synchronized void enqueue(String item) {
        Node oldlast = last;
        last = new Node();
        last.item = item;
        last.next = null;
        if (isEmpty())
            first = last;
        else
            oldlast.next = last;
    }

    public synchronized String dequeue() {
        String item = first.item;
        first = first.next;
        if (isEmpty())
            last = null;
        return item;
    }
}

class FolderScan extends MultiThreadingSearch implements Runnable {

    FolderScan(String path, String whatFind) {
        super(path, whatFind);
    }

    @Override
    public void run() {
        findFiles(path);
    }

    ArrayListOfFiles findFiles(String path) {
        File root = new File(path);
        File[] list = root.listFiles();
        for (File titleName : list) {
            if (titleName.isDirectory()) {
                findFiles(titleName.getAbsolutePath());
            } else {
                if (titleName.getName().toLowerCase().endsWith((".txt"))) {
                    textFiles.enqueue(titleName.getName());
                }
            }
        }

        return textFiles;
    }

}

class FileScan extends MultiThreadingSearch implements Runnable {
    Scanner scanner = new Scanner((Readable) textFiles);
    Set<String> words = new HashSet<String>();
    int matches = 0;

    FileScan(String file, String whatFind) {
        super(file, whatFind);
        Thread wordFind = new Thread();
        wordFind.start();
    }

    @Override
    public void run() {
        while (scanner.hasNext()) {
            String word = scanner.next();
            words.add(word);
        }

        if (words.contains(this.whatFind)) {
            System.out.println("File:" + this.path);
            matches++;
        }

        System.out.println(matches);
    }

}

public class MultiThreadingSearch {
    String path;
    String whatFind;

    ArrayListOfFiles textFiles;

    MultiThreadingSearch(String path, String whatFind) {
        this.path = path;
        this.whatFind = whatFind;
        this.textFiles = new ArrayListOfFiles();

        Thread pathFind = new Thread(new FolderScan(path, whatFind));
//      pathFind.start();

        if (!textFiles.isEmpty()) {
            @SuppressWarnings("unused")
            FileScan fileScan = new FileScan(textFiles.dequeue(), whatFind);
        }

    }

    // ask user about input
    public static void askUserPathAndWord() {

        BufferedReader bufferedReader = new BufferedReader(
                new InputStreamReader(System.in));
        String path;
        String whatFind;
        try {
            System.out.println("Please, enter a Path and Word"
                    + "(which you want to find):");
            System.out.println("Please enter a Path:");
            path = bufferedReader.readLine();
            System.out.println("Please enter a Word:");
            whatFind = bufferedReader.readLine();

            if (path != null && whatFind != null) {
                new MultiThreadingSearch(path, whatFind);
                System.out.println("Thank you!");
            } else {
                System.out.println("You did not enter anything");
            }

        } catch (IOException | RuntimeException e) {
            System.out.println("Wrong input!");
            e.printStackTrace();
        }
    }


    public static void main(String[] args) {
        askUserPathAndWord();
    }
}

I got Exception in thread "main" java.lang.StackOverflowError from this code.
How able to solve this task?

Thanks,
Nazar.

Answer 1

Check BlockingQueue it does exactly what you need. Thread can block until some other thread add new item to queue.
As to how decompose you system. I’d do following:

• Create class for searching txt files in path. It implements Runnable. You pass path and queue to it. And it searches path for txt files and adds them to the queu.
• Create class for searching file content. It implements Runnable. You pass whatFind and queue to it and it takes new file from queue and checks it’s content.

Something like:

BlockingQueue<File> queue = new LinkedBlockingQueue<File>();
String path = ...;
String whatFind = ...;
FolderScan folderScan = new FolderScan(path, queue);
FileScan fileScan = new FileScan(whatFind, queue);

Executor executor = Executors.newCachecThreadPool();
executor.execute(folderScan);
executor.execute(fileScan);

If you want FileScan to wait until FolderScan adds something to the queue you can use take method:

BlockingQueue<File> queue;
File toProcess = queue.take(); // this line blocks current thread (FileScan) until someone adds new item to the queue.