public class Main {
    Integer i = 1;

    public Main() {

    }

    public static void main(String args[]) throws InterruptedException {
        final Main t = new Main();
        synchronized (t.i) {
            Thread thread = new Thread() {
                public void run() {
                    for (;;) {
                        synchronized (t.i) {
                            System.out.println("a:" + (++t.i));
                            try {
                                Thread.sleep(100);
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                            t.i.notifyAll();
                        }
                    }
                }
            };
            thread.start();
            t.i.wait();
            System.out.println("b:" + (++t.i));
        }
    }
}

a:2
Exception in thread "Thread-0" java.lang.IllegalMonitorStateException
    at java.lang.Object.notifyAll(Native Method)
    at Main$1.run(Main.java:21)