wait() and notify() method , always IllegalMonitorStateException is happen and tell me current Thread is not Owner Why?

package pkg_1;

public class ExpOnWaitMethod extends Thread {

    static Double x = new Double(20);


    public static void main(String[] args) {

        ExpOnWaitMethod T1 = new ExpOnWaitMethod();
        ExpOnWaitMethod T2 = new ExpOnWaitMethod();

        T1.start();

        T2.start();

    }

    public void run() {

        Mag mag = new Mag();

        synchronized (x) {

            try {
                for (int i = 1; i < 10; i++) {
                    mag.nop(Thread.currentThread());
                    x = i * 2.0;

                }

            } catch (InterruptedException e) {

                e.printStackTrace();
            }

        }
    }

}

class Mag {
    char ccc = 'A';

    public void nop(Thread thr) throws InterruptedException {

        System.out.print(ccc + " ");
        ccc++;

        if (thr.getState().toString().equalsIgnoreCase("runnable"))
            Thread.currentThread().wait();
        //thr.notify();
    }
}

Answer

You need to hold the lock on the object you want to wait on (you can only call it within a synchronized block).

Also, calling wait on a Thread is very unusual and probably not what you want.

I am not sure what you are trying to do, but could you be confusing wait with sleep?

If you want to wait for another thread to finish, that would be anotherThread.join().