What is the difference when using standalone wait() method and using any instance of a class ( obj.wait() )

Question

I am trying to execute 2 threads in interleaved manner. One is printing odd numbers and other is printing even numbers to the console (<=100). I want the output to be : 1 2 3 4 5 6 7 8 9 10 .... and so on. i.e they should execute one after another in order.

Here is the code for this:

class Monitor{
    boolean flag = false;
}

class OddNumberPrinter implements Runnable{
    Monitor monitor;
    OddNumberPrinter(Monitor monitor){
        this.monitor = monitor;
    }

    @Override
    public void run() {
        for (int i = 1; i<100; i+=2){

            synchronized (monitor){
                while(monitor.flag == true){
                    try {
                        wait();
                    } catch (Exception e) {}
                }
                System.out.print(i+"  ");
                monitor.flag = true;
                notify();
            }
        }
    }
}
class EvenNumberPrinter implements Runnable{
    Monitor monitor;
    EvenNumberPrinter(Monitor monitor){
        this.monitor = monitor;
    }

    @Override
    public void run() {
        for (int i = 2; i<101; i+=2){
            synchronized (monitor){
                while (monitor.flag == false){
                    try {
                        wait();
                    } catch (Exception e) {}
                }
                System.out.print(i + "  ");
                monitor.flag = false;
                notify();
            }
        }
    }
}
public class InterleavingThreads {

    public static void main(String[] args) throws InterruptedException {
        Monitor monitor = new Monitor();
        Thread t1 = new Thread(new OddNumberPrinter(monitor));
        Thread t2 = new Thread(new EvenNumberPrinter(monitor));

        t1.start();
       t2.start();
    }
}

When I standalone use wait() and notify(), it throws exception, but using monitor.wait() and monitor.notify() gives the correct output. Please explain the difference in these two.