java - When to use mock objects in unit tests

Question

I'm aware that there are many questions about mocking and testing, but I didn't find any that helped me perfectly, so I still have problems understanding the follwing:

Please correct me if I got this wrong, but as far as I see, unit tests are used to test the business logic of one particular class in isolation and if there are any objects needed from the outside they will be mocked. So for example if I have a management system for citizens of a simple city that adds citizens to a list and returns the citizen by their names (Assumtion: citizens consist of only a few basic personal information), like this:

public class ProcessClass {

    ArrayList<Citizen> citizenList = new ArrayList<Citizen>();

    public void addCitizen(Citizen citizen) {
        citizenList.add(citizen);
    }

    public Citizen getByName(String name) {
        for (Citizen c : citizenList) {
            if (c.getName().equals(name)) {
                return c;
            }
        }
        return null;
    }

}

If now I want to unit test my ProcessClass do I consider Citizen as an external feature that has to be mocked, or do I simply just create a Citizen for test purposes? If they are mocked, how would I test the method to get the object by its name, since the mock object is not containing the parameters?

Answer 1

As you're writing new code (along with the new unit tests) or refactoring existing code, you want to be able to run the unit tests over and over to be reasonably confident existing functionality was not broken. Therefore, the unit tests must be stable and fast.

Suppose the class to be tested depends on some external resource such as a database. You make a code change and the unit tests are suddenly failing. Did the unit tests break because of a bug you just introduced, or because the external resource is not available? There is no guarantee the external resource will always be available so the unit tests are unstable. Mock the external resource.

Also, connecting to an external resource can take too much time. When you eventually have thousands of tests which connect to various external resources, the milliseconds to connect to the external resource add up, which slows you down. Mock the external resource.

Now add a CI/CD pipeline. During the build the unit tests fail. Is the external resource down or did your code change break something? Perhaps the build server doesn't have access to an external resource? Mock the external resource.