We design the two interfaces that we will implement. When we design interfaces, we focus on the functionality first. Formatting and protocol come later. Interfaces generally should be simple and, at the same time, should accommodate the future change. This is a hard problem because we cannot see the future. Business, logistics, and all other experts may see some part of the future: how the world will change and what it will impose on the operation of the company and, especially, on the interface we provide for our partners.

The stability of an interface is of utmost importance because the partners are outside entities. We cannot refactor the code they use. When we change a Java interface in our code, the compiler will complain at all the code locations where the change should be followed. In case of an interface that is used outside of our realm, this is not the case. Even if it is only a Java interface that we publish as open source on GitHub, we should be prepared...