Agree with that. If you want to unit test something, you should be able to test the outputs for a given set of inputs, plus possibly verify some interactions, i

Agree with that. If you want to unit test something, you should be able to test the outputs for a given set of inputs, plus possibly verify some interactions, i.e. if I call chargeCustomersCreditCard(BillingInfo billing, Invoice invoice), then there will be exactly one call to myCreditCardProcessingMock.chargeTheCard(String creditCardNumber, Decimal amount), where creditCardNumber == billing.creditCardNumber and amount == invoice.totalAmount.