Message d'erreur

Bonjour,
voici mon code java si je veux executer sous dos java Calcu.class ce message sera affiché
Exception in thread "main" java.lang.NoClassDefFoundError: Essai/class
voici le code
public class Calcul
{
public static void main(String args[])
{
Shared shared = new Shared();
CustomThread thread1 = new CustomThread(shared,"1");
CustomThread thread2 = new CustomThread(shared,"2");
CustomThread thread3 = new CustomThread(shared,"3");
CustomThread thread4 = new CustomThread(shared,"4");
CustomThread thread5 = new CustomThread(shared,"5");
try
{
thread1.join();
thread2.join();
thread3.join();
thread4.join();
thread5.join();
} catch(InterruptedException e){}
}
}
class CustomThread extends Thread
{
Shared shared;
public CustomThread(Shared shared, String string)
{
super(string);
this.shared=shared;
start();
}
public void run()
{
synchronized(shared)
{
shared.doWork(Thread.currentThread().getName());
}
}
}
class Shared
{
void doWork(String string)
{
System.out.println("demmarrage de" + string);
try
{
Thread.sleep((long) (Math.random() * 500));
} catch (InterruptedException e) {}
System.out.println("fin de" + string);
}
}
aidez moi svp.merci