Sharing object between multiple threads

Priyatam Roy :

Can't get the correct value of count in writer thread. It is always 1 in the writer thread even though it is changing in reader thread.

public class ReaderWriter1 {

    public static void main(String args[]) {

        Semaphore rs = new Semaphore(1);
        Integer count = new Integer(0);

        Thread r1 = new Thread(new Reader("Reader 1", rs, count++));
        Thread w1 = new Thread(new Writer("Writer 1", count));
        w1.start();
        r1.start();
    }
}

class Reader implements Runnable {

    String tName;
    Semaphore rs;
    Integer count;

    Reader(String tName, Semaphore rs, Integer count) {
        this.tName = tName;
        this.rs = rs;
        this.count =  count;
    }

    @Override
    public void run() {
        try {
            read();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    void read() throws InterruptedException {
        while(true) {

            rs.acquire();
            count++;
            rs.release();
            System.out.println("Count in reader: " + count);
            Thread.sleep(1000);
        }
    }

}

class Writer implements Runnable {

    String tName;
    Integer count;

    Writer(String tName, Integer count) {
        this.tName = tName;
        this.count =  count;
    }

    @Override
    public void run() {

        try {
            write();
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }

    }

    void write() throws InterruptedException {
        while(true) {
            System.out.println("Count in writer: " + count);
            Thread.sleep(1000);
        }
    }

}

Output:

Count in writer: 1
Count in reader: 1
Count in writer: 1
Count in reader: 2
Count in reader: 3
Count in writer: 1
Count in writer: 1
Count in reader: 4
Count in writer: 1
Count in reader: 5
Count in writer: 1
Count in reader: 6
Count in writer: 1
Count in reader: 7
Count in reader: 8
Count in writer: 1
Count in reader: 9
Count in writer: 1

Please let me know what is wrong with my code.

M Anouti :

The code is not sharing the Integer instance. count++ is equivalent to:

count = Integer.valueOf(count.intValue() + 1);

i.e. so you re-assign a new instance to the local variable count. The instance itself is not changed (indeed Integer is an immutable type).

In multithreaded scenarios, it might be better to use an AtomicInteger.

Side note: you almost always should not invoke the Integer constructor, always use Integer.valueOf(int).