Computer Science Foundations

Can anyone tell me what the hell is wrong with this Diffy code?

Diffy d = new Diffy (n1, n2, n3, n4);
d.printDiffy();
System.out.println();
while (d.isZero() == false)
{
  d.next(); 
  d.printDiffy();
  System.out.println();                                                         
}