Mocking a class vs. mocking its interface

Question

For a unit test, I need to mock several dependencies. One of the dependencies is a class which implements an interface:

public class DataAccessImpl implements DataAccess {     ... } 

I need to set up a mock object of this class which returns some specified values when provided with some specified parameters.

Now, what I'm not sure of, is if it's better to mock the interface or the class, i.e.

DataAccess client = mock(DataAccess.class); 

vs.

DataAccess client = mock(DataAccessImpl.class); 

Does it make any difference in regard to testing? What would be the preferred approach?

Answer 1

It may not make much difference in your case but the preferred approach is to mock interface, as normally if you follow TDD (Test Driven Development) then you could write your unit tests even before you write your implementation classes. Thus even if you did not have concrete class DataAccessImpl, you could still write unit tests using your interface DataAccess.

Moreover mocking frameworks have limitations in mocking classes, and some frameworks only mock interfaces by default.