Java MultiThreading skips loop and gives wrong result [duplicate]

Question

Java MultiThreading skips loop and gives wrong result

package Threading;

class DemoThread extends Thread{   //Thread Class   
    static int count=0;   // variable incremented by both the threads

    public DemoThread(String name) {
        // TODO Auto-generated constructor stub
        super(name);
    }

    public void run() { 
        for(int i=0;i<100000;i++) {
            count++;
            System.out.println(Thread.currentThread()+"Count"+count);  // print thread operating on count variable
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }           
        }       
    }   
}


public class MyThreadClass {

    public static void main(String[] args) {
        // TODO Auto-generated method stub      
        DemoThread t1=new DemoThread("T1");
        DemoThread t2=new DemoThread("T2");
        t1.start();
        t2.start();
        try {
            t1.join();
            t2.join();  //allowing both the threads to complee before main thread
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("Main Thread ends"+DemoThread.count);  //final value of count        
    }
}

The final value of count should be 199998 but it is not giving the desired result. Why the threads are missing the loops ???

Answer 1

It happened because Thread T1 and T2 will update count at the same time (concurrency) like that:

Thread[T1,5,main]Count10
Thread[T2,5,main]Count10
Thread[T1,5,main]Count12
Thread[T2,5,main]Count12
Thread[T2,5,main]Count14
Thread[T1,5,main]Count14
Thread[T1,5,main]Count15
Thread[T2,5,main]Count16

You should use AtomicInteger

And update your code:

static int count=0; to static AtomicInteger count= new AtomicInteger();

count++; to count.incrementAndGet();