Synchronized method in java does not work as indended

Question

I want to learn how to use the synchronized method in java and implemented the following code.

public class checkThread {
    volatile int i = 0;
    
    public void increment() {
        i++;
    }
}
public class TestSync extends checkThread{
    public static void main(String[] args) {
        checkThread ct1 = new checkThread();
        Object iLock = new Object();
        for(int i = 0 ; i < 10 ; i++) {
            extracted(ct1, iLock);      
        }
    }    
    private static void extracted(checkThread ct1, Object iLock) {
        synchronized (iLock) {
            Thread t1 = new Thread(new Runnable() {
                @Override
                public void run() {
                    for(int a = 0; a < 1000; a++) {
                        ct1.increment();
                    }
                }
            });
            t1.start();
        }
        synchronized (iLock) {
            Thread t2 = new Thread(new Runnable() {
                @Override
                public void run() {
                    for(int a = 0; a < 1000; a++) {
                        ct1.increment();
                    }
                }
            });
            t2.start();
        }
        synchronized (iLock) {
            System.out.println(ct1.i);
        }
    }
}

However the output I get is not at all synchronized!

1000 2000 4000 6000 8000 9000 11575 13575 15575 17459

Why am I getting such an output and not the desired value of i in the multiples of 1000?

Answer 1

If you are trying to make sure that the run() method will be completed by one thread before the other one start; then you need to synchronize the content inside the run method, not the thread creation part.

In your specific example, you need to have an object that can be accessed by both the threads, and then acquire the lock of that object.

Let me add an example below by changing your code; but this may not be the best way; just trying to explain the point.

checkThread iLock = new checkThread();

public void someMethod() {

    Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {
            synchronized (iLock) {            
                for(int a = 0; a < 1000; a++) {
                    ct1.increment();
                }
            }
        });
        t1.start();
    }

    Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
            synchronized (iLock) {            
                for(int a = 0; a < 1000; a++) {
                    ct1.increment();
                }
            }
        });
        t2.start();
    }
    
    synchronized (iLock) {
        System.out.println(ct1.i);
    }
}