Bla.java

public class CustomerServiceTest{
  @Test
  public void shouldGenerateInvoices() {
    // mocking CustomerDAO
    CustomerDAO customerDao = Mockito.mock(CustomerDAO.class);
    Mockito.when(
        customerDao.findCustomerById(1))
        .thenReturn(new Customer(1, "Customer1"));
    
    // instantiating the CUT, and passing
    // a mock as dependency
    CustomerService cut = new CustomerService(customerDao);
    cut.generateInvoices();
    
    // asserts here
  }
}