AtomicReference 无法避免 java 多线程中的竞争条件

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");
    }
}

Answer 1

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.

Answer 2

++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