Mocking a class vs. mocking its interface

JavaUnit TestingJunitMocking

Java Problem Overview


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?

Java Solutions


Solution 1 - Java

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.

Solution 2 - Java

In most cases technically there is no difference and you may mock as class so an interface. Conceptually it is better to use interfaces because of better abstraction.

Solution 3 - Java

It depends. If your code depends on the class and not on the interface you must mock the class to write a valid unit test.

Solution 4 - Java

You should mock the interface since it will help ensure you are adhering to Liskov Substitution Principal (https://stackoverflow.com/a/56904/3571100).

Solution 5 - Java

If you only use it through interface and it's not a partial mock, there is no difference other than your inner feeling. Mocking the class will also mock non-used public method if the class has them, but that is not a big deal to consider.

Attributions

All content for this solution is sourced from the original question on Stackoverflow.

The content on this page is licensed under the Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license.

Content TypeOriginal AuthorOriginal Content on Stackoverflow
QuestionhelpermethodView Question on Stackoverflow
Solution 1 - JavaKuldeep JainView Answer on Stackoverflow
Solution 2 - JavaAlex NikolaenkovView Answer on Stackoverflow
Solution 3 - JavaonofView Answer on Stackoverflow
Solution 4 - JavajordanView Answer on Stackoverflow
Solution 5 - JavaAlex AbdugafarovView Answer on Stackoverflow