One of the hardest things for people to understand in OO design is the “Tell Don’t Ask” principle, which encourages you to tell an object to do something rather than rip data out of an object to do it in client code.

Mockists say that using mockist testing helps promote this and avoid the getter confetti that pervades too much of code these days.

Classicists argue that there are plenty of other ways to do this.

Martin Fowler, in Mocks Aren’t Stubs