Is it recommended to mock concrete class?

Question

Most of the examples given in mocking framework website is to mock Interface. Let say NSubstitute that I\'m currently using, all their mocking examples is to mock interface.

Answer 1

Supposed we have:

class Foo {
    fun bar() = if (someCondition) {
        “Yes”
    } else {
        “No”
    }
}

There’s nothing preventing us to do the following mocking in the test code:

val foo = mock()
whenever(foo.bar()).thenReturn(“Maybe”)

The problem is it is setting up incorrect behavior of class Foo. The real instance of class Foo will never be able to return “Maybe”.