CODEWITHSUNDEEP
javamultithreadingconcurrency
msk
msk

Reputation: 171

getting inconsistent/wrong output in the program Multi -Threading java

/*
This should always produce 0 as output since all three methods increment(), decrement(), value()  are thread safe(synchronized).  but it is returning 1
*/

class Counter implements Runnable {
    private int c = 0;

    public  synchronized void increment() {
        c++;
    }
    public synchronized void decrement() {
        c--;
    }
    public synchronized int value() {
        return c;
    }
    public void run() {
        try {
            this.increment();
            Thread.sleep(1000);
            this.decrement();
            Thread.sleep(1000);
            this.increment();
            Thread.sleep(1000);
            this.decrement();
            Thread.sleep(1000);
        }
        catch (InterruptedException e){
            return;
        }
    }
    public static void main(String args[]) throws InterruptedException {
       Counter c =  new Counter();
       new Thread(c).start();
       new Thread(c).start();
       System.out.println(c.value());
    }

}

Upvotes: 0

Views: 679

Answers (4)

luke
luke

Reputation: 14788

like everyone else said you need to make sure that the treads have finished executing, to do that you need to call join. for example

public static void main(String args[]) throws InterruptedException {
   Counter c =  new Counter();
   Thread t1 = new Thread(c).start();
   Thread t2 = new Thread(c).start();
   t1.join();
   t2.join();
   System.out.println(c.value());
}

that should run correctly

Upvotes: 5

erickson
erickson

Reputation: 269637

There's nothing to control when the main thread is calling value(). It will run as soon as it can acquire a lock on c, even though the other threads are still running.

If you want to wait until the threads are done, call join() on them.

Upvotes: 4

user124493
user124493

Reputation:

You're not waiting for the threads to complete running, so the result is that the value of c is printed at whatever it is at that second. I bet if you tried it 1000 times, there would be times when it wasn't 1.

IBM has a fair tutorial on the situation you're encountering: http://publib.boulder.ibm.com/infocenter/iseries/v5r3/index.jsp?topic=/rzahw/rzahwex3rx.htm

Upvotes: 1

starblue
starblue

Reputation: 56752

You are reading the value before the threads have finished execution, so it may be well different from zero.

Upvotes: 1

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