i get an exception when i run my Main Class :[exception on Thread's](https://i.stack.imgur.com/4lNa0.png)

i using run method, and it look like this : 

```
    @Override
public synchronized void run() {
    // TODO Auto-generated method stub
    int randomNum = ThreadLocalRandom.current().nextInt(1, 4);
    Student studentThread = new Student();
    Inlay inlayThread = new Inlay();
    studentThread.setId(counter);
    inlayThread.setInlayId(randomNum);
    Course courseThread = new Course();
    courseThread.setInlayD(inlayThread);
    courseThread.setStudentD(studentThread);
    this.arrCourse.add(courseThread);
    System.out.println(courseThread.toString());
    counter++;
}

```

and my Main :

```
    Course c = new Course();
    for(int i = 0 ; i<200;i++)
    {
        Thread t = new Thread(c);
        t.start();
    }
    c.toString();

```

what is tihs exception say ?

thanks !