Encapsulation for Design by Contract

The Three Reasons for Data Encapsulation "It’s not that data encapsulation does not separate interface from implementation, or that separating interface from implementation is unimportant. It’s just that this is by far the least important reason for data encapsulation...Data encapsulation allows programmers to enforce class invariants, preconditions, and postconditions."

"The I in API stands for interface, and interfaces are for people, not just machines. APIs can be complex, confusing things. The less there is of it, the better. The smaller and simpler the API is, the easier it is to learn and use; the more likely it is that the API will be used correctly."