Java Multithreading: An Unexpected Result

Question

I am working on a corporate application. I am encountering some problems when running the application in a multi-threaded environment. I am writing a program in which there is a variable whose value is updated (increased) at a very high speed (for example, 10,000 updates / year). The loop is executed for certain iterations, and the value of the variable is increased and stored in the HashMap. As soon as the loop finishes and the value of the HashMap variable is printed. I get an unexpected variable value.

Here is a demo program (please read the comments for a better understanding):

class test implements Runnable {

    static ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
    static AtomicInteger value_to_be_incremented_stored = new AtomicInteger(0); // variable whose value to be updated
    static AtomicInteger i = new AtomicInteger(0);  // this runs the loop

    @Override
    public void run() {

        for (i.set(0); i.get() < 100000; i.incrementAndGet()) {
            /*
                This loop should run 100000 times and when loop terminates according to me value of variable 
                "value_to_be_incremented_stored" should be 100000 as its value is incremented 
                100000 times the loop also runs 100000 times. 
            */
            System.out.println("Thread > " + Thread.currentThread() + "  " + value_to_be_incremented_stored.incrementAndGet());
            map.put("TC", value_to_be_incremented_stored.intValue());
        }

        System.out.println("Output by Thread  " + Thread.currentThread() + "     " + map.toString());
    }

    public static void main(String[] args) {

        test t1 = new test();
        Thread thread1 = new Thread(t1);
        thread1.setName("Thread 1");

        Thread thread2 = new Thread(t1);
        thread2.setName("Thread 2");

        Thread thread3 = new Thread(t1);
        thread3.setName("Thread 3");

        Thread thread4 = new Thread(t1);
        thread4.setName("Thread 4");

        thread1.start();
        thread2.start();
        thread3.start();
        thread4.start();

    }
}

Output (it changes):

Problem : I run the loop 100000 times (i.get() < 100000), and then the value of the variable value_to_be_incremented_storedbecomes more than 100000.

Answer 1

. - for , , . , :

for ( ; i.incrementAndGet() < 100000;  ) {

. , ( ), . , , , , , .

synchronized( lock ) { 
    value_to_be_incremented_stored.incrementAndGet();
    map.put("TC", value_to_be_incremented_stored.intValue());
}

, - < 99999. <=, .

(, , i.set(0) for . , .)

class ThreadTestX implements Runnable {

    static ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
    static AtomicInteger value_to_be_incremented_stored = new AtomicInteger(0); // variable whose value to be updated
    static AtomicInteger i = new AtomicInteger(0);  // this runs the loop
    static final Object lock = new Object();

    @Override
    public void run() {

        for ( ; i.incrementAndGet() <= 100000;  ) {
            /*
                This loop should run 100000 times and when loop terminates according to me value of variable 
                "value_to_be_incremented_stored" should be 100000 as its value is incremented 
                100000 times the loop also runs 100000 times. 
            */
            synchronized( lock ) {
                value_to_be_incremented_stored.incrementAndGet();
    //            System.out.println("Thread > " + Thread.currentThread() + 
    //                 "  " + value_to_be_incremented_stored.get());
                map.put("TC", value_to_be_incremented_stored.intValue());
            }
        }

        System.out.println("Output by Thread  " + Thread.currentThread() 
                + "     " + map.toString());
    }

    public static void main(String[] args) {

        ThreadTestX t1 = new ThreadTestX();
        Thread thread1 = new Thread(t1);
        thread1.setName("Thread 1");

        Thread thread2 = new Thread(t1);
        thread2.setName("Thread 2");

        Thread thread3 = new Thread(t1);
        thread3.setName("Thread 3");

        Thread thread4 = new Thread(t1);
        thread4.setName("Thread 4");

        thread1.start();
        thread2.start();
        thread3.start();
        thread4.start();

    }
}

:

run:
Output by Thread  Thread[Thread 4,5,main]     {TC=100000}
Output by Thread  Thread[Thread 3,5,main]     {TC=100000}
Output by Thread  Thread[Thread 1,5,main]     {TC=100000}
Output by Thread  Thread[Thread 2,5,main]     {TC=100000}
BUILD SUCCESSFUL (total time: 0 seconds)

---

: , , , , . , : i, , , . . , :

@Override
public void run() {

    for ( ;;  ) {
        synchronized( lock ) {
            if( i.incrementAndGet() <= 100000 ) {
                value_to_be_incremented_stored.incrementAndGet();
                map.put("TC", value_to_be_incremented_stored.intValue());
            }
            else
                break;
        }
    }
    System.out.println("Output by Thread  " + Thread.currentThread() 
            + "     " + map.toString());
}

AtomicInteger, , , (- - ), .

Answer 2

" " .

№1 - i.set( 0 ), tsolakp. , , - 100000. , , - 100000.

, , 3, 10 20 . 100003, 100009 100019 . . ?

, , i 99999, i.get() < 100000 , . i.incrementAndGet() i.get() < 1000000;, .

, i .

Answer 3

, run, reset i for, i.set(0).

: reset - , . , for, i. , , value_to_be_incremented_stored 3 i .