Why is Java synchronized not working as expected?

Question

I\'m trying to figure out how synchronized methods work. From my understanding I created two threads T1 and T2 that will call the same method <

Answer 1

try this :

public class Main {
    public static void main(String[] args) {
        A a = new A();
        Thread t1 = new Thread(a);
        Thread t2 = new Thread(a);
        t1.setName("T1");
        t2.setName("T2");
        t1.start();
        t2.start();
    }
}

 class B {
    public synchronized void addNew(int i){
        Thread t = Thread.currentThread();
        for (int j = 0; j < 5; j++) {
            System.out.println(t.getName() +"-"+(j+i));
        }
    }
}

 class A extends Thread {
    private B b1 = new B();

    @Override
    public void run() {
        b1.addNew(100);
    }
}

Answer 2

Both A objects have their own B object. You need them to share a B so the synchronization can have an effect.

Answer 3

Each A instance has its own B instance. The method addNew is an instance method of B. Therefore, the lock acquired implicitly during calls to addNew is the lock on the receiver B instance. Each thread is calling addNew on a different B, and therefore locking on different locks.

If you want all B instances to use a common lock, create a single shared lock, and acquire it in the body of addNew.