AtomicReference not working to avoid race condition in java multi threading

Question

I have a "User.java" class that has Integer variable count initially set to 0. In another class "ThreadDemo.java" I have set the User object in AtomicReference. This "userRef" object is shared by "1000" threads and in each thread I am incrementing the "count" value by 1. But here i am not getting expected answer 1000.Each time I execute "count" variable gives different value like 1000,1002 etc.. But if i try to use synchronization or AtomicInteger then it works. So my question is ,am i using AtomicReference concept properly to resolve race condition in this case.If yes, then Why I it is failing in this case? . Please find the piece of code below: User.java:-

public class User {

     public Integer count=0; 

}

ThreadDemo.java:-

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicReference;

public class ThreadDemo {

    static AtomicReference<User> userRef = new AtomicReference<User>(new User());

    public static void main(final String[] arguments) throws InterruptedException {

        System.out.println("main thread started");

        List<Thread> listThread = new ArrayList<>();

        for (int i = 1; i <= 1000; i++) {
            listThread.add(new Thread() {
                @Override
                public void run() {

                    boolean flag = false;

                    while (!flag) {
                        User prevValue = userRef.get();
                        User newValue = new User();
                        newValue.count = ++prevValue.count;
                        flag = userRef.compareAndSet(prevValue, newValue);
                    }

                }
            });
        }

        for (Thread t : listThread) {
            t.start();
        }

        for (Thread t : listThread) { 
            t.join();
        }

        System.out.println("user count:" + userRef.get().count);
        System.out.println("main thread finished");
    }
}

I'd bet that to start with, instead of `newValue.count = ++prevValue.count;`, you want `newValue.count = prevValue.count + 1;`, which doesn't modify `prevValue`. — Louis Wasserman, Mar 28 '18 at 17:10

score 0 · Answer 1 · answered Apr 01 '18 at 14:04

I not got the answer for this.It is giving wrong value because of "newValue.count = ++prevValue.count". Here the pre increment operator chances the value of "prevValue"., Instead write this statement as "newValue.count = prevValue.count+1".It works fine then.

score 0 · Answer 2 · answered Apr 21 '22 at 21:24

++prevValue.count changed the value

prevValue.count: 1
after ++prevValue.count
newValue.count, prevValue.count: 2
compareAndSet failed, matched while loop condition
prevValue.count: 2
after ++prevValue.count
newValue.count, prevValue.count: 3
compareAndSet success

AtomicReference not working to avoid race condition in java multi threading

2 Answers2