Question

I’ve got a piece of code like this:

    class Lekcja {
        Calendar _date;
        public Lekcja() {
            _date = Calendar.getInstance();
        }
        public Calendar getDate() {
            return _date;
        }
    }

And I want to test it using JUnit4 and Mockito, instead of using real Calendar object I want to put a mock object there. Could you please tell me how to do it?

Answer 1

Mocking value objects is rarely a good idea. You should mock behaviour, not data. I guess what you really want to achieve is to be able to use arbitrary date in tests (which is a good idea).

The pattern I am using successfully in such cases is a fake system clock (examples from that site):

public interface TimeSource {
  long currentTimeMillis();
} 

And two implementations, real:

public final class RealSource implements TimeSource {

  public long currentTimeMillis() {
    return System.currentTimeMillis();
  }

}

and fake for testing:

public final class FakeSource implements TimeSource {

  public long currentTimeMillis() {
    return //...whatever you want
  }

}

I find it convenient to make currentTimeMillis() static in a helper method and use static field pointing to current TimeSource.

Finally, much simpler approach is to pass the date directly:

class Lekcja {
    Calendar _date;
    public Lekcja(Calendar date) {
        this._date = date;
    }
    //...
}

See also

• Unit testing time-based logic in Java