count the sum of digits from the file threeds-CodePudding

My task is to read three files from three threads using the synchronized keyword. This is my code that I am using, however, when I print "SumOfTheFiles", it is printing 0. What am I doing wrong ?

    public class Dispatcher {

    public static void main(String[] args) {

        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.execute(new Thread(new SumOfDigits(new File("text.txt"))));
        executorService.execute(new Thread(new SumOfDigits(new File("text2.txt"))));
        executorService.execute(new Thread(new SumOfDigits(new File("text3.txt"))));

        executorService.shutdown();
        System.out.println(SumOfDigits.SumOFTheFiles);
    }

}

class SumOfDigits implements Runnable {
    File file;
    static int SumOFTheFiles = 0;

    public SumOfDigits(File file) {
        this.file = file;
    }

    @Override
    public void run() {
        SumOFTheFiles   = sum();
    }

    synchronized int sum() {
       int sum = 0;
       try(Scanner sc = new Scanner(file)){
           while(sc.hasNext()){
               sum  = Integer.parseInt(sc.next());
           }
       }catch (Exception e){
           e.printStackTrace();
       }
        return sum;
    }
}

CodePudding user response：

The issue here is that you not are enforcing synchronized access to the SumOfTheFiles variable. You are calculating the sum in a synchronized block, however, you are accessing the variable in the asynchronous run methods. To prevent this, update the SumOfTheFiles variable in the sum method. Example:

public class ThreadTest {

    public static void main(String[] args) {
        Thread s1 = new Thread(new SumOfDigits(new File("f1.txt")));
        Thread s2 = new Thread(new SumOfDigits(new File("f2.txt")));
        Thread s3 = new Thread(new SumOfDigits(new File("f3.txt")));

        // Run the threads
        s1.start();
        s2.start();
        s3.start();

        // Wait for completion
        try {
            s1.join();
            s2.join();
            s3.join();
        } catch (Exception e) {
            e.printStackTrace();
        }

        System.out.println(SumOfDigits.SumOFTheFiles);
    }

}

class SumOfDigits implements Runnable {
    static int SumOFTheFiles = 0;
    File aFile;

    @Override
    public void run() {
        this.sum();
    }

    public SumOfDigits(File aFile) {
        this.aFile = aFile;
    }

    // New sum method 
    synchronized void sum() {
        int sum = 0;

        try {
            Scanner sc = new Scanner(this.aFile);
            while(sc.hasNextInt()){
                sum  = sc.nextInt();
            }
            sc.close();
        } catch (Exception e) {
            e.printStackTrace();
        }

        SumOFTheFiles  = sum;
     }
}

The output of this code is always 74 given f1 contains "9 9 9 9 9", f2 contains "8 8 8", and f3 contains "3 2".