Question

I’d like to test an abstract class. Sure, I can manually write a mock that inherits from the class.

Can I do this using a mocking framework (I’m using Mockito) instead of hand-crafting my mock? How?

Answer 1

The following suggestion lets you test abstract classes without creating a "real" subclass – the Mock is the subclass and only a partial mock.

Use Mockito.mock(My.class, Answers.CALLS_REAL_METHODS), then mock any abstract methods that are invoked.

Example:

public abstract class My {
  public Result methodUnderTest() { ... }
  protected abstract void methodIDontCareAbout();
}
 
public class MyTest {
    @Test
    public void shouldFailOnNullIdentifiers() {
        My my = Mockito.mock(My.class, Answers.CALLS_REAL_METHODS);
        Assert.assertSomething(my.methodUnderTest());
    }
}

Note: The beauty of this solution is that you do not have to implement the abstract methods. CALLS_REAL_METHODS causes all real methods to be run as is, as long as you don’t stub them in your test.

In my honest opinion, this is neater than using a spy, since a spy requires an instance, which means you have to create an instantiable subclass of your abstract class.