public boolean accountIsDelinquent(Customer customer) {
   Date today = new Date();
   Specification delinquentSpec =
      new DelinquentInvoiceSpecification(today);
   Iterator it = customer.getInvoices().iterator();
   while (it.hasNext()) {
      Invoice candidate = (Invoice) it.next();
   if (delinquentSpec.isSatisfiedBy(candidate)) return true;
   }
   return false;
}
