How to use CallsRealMethodsTest class of org.mockito.internal.stubbing.answers package

Note that documentToPojo takes a Class as its second argument. any(Foo.class) returns an argument of type Foo, not of type Class<Foo>, whereas eq(WorkItemDTO.class) should return a Class<WorkItemDTO> as expected. I'd do it this way:

when(mongoUtil.documentToPojo(
    Mockito.any(Document.class),
    Mockito.eq(WorkItemDTO.class))).thenReturn(...);

Technically speaking both "mocks" and "spies" are a special kind of "test doubles".

Mockito is unfortunately making the distinction weird.

A mock in mockito is a normal mock in other mocking frameworks (allows you to stub invocations; that is, return specific values out of method calls).

A spy in mockito is a partial mock in other mocking frameworks (part of the object will be mocked and part will use real method invocations).

Further reading has led me to try using ArgumentCaptors and the following works, although much more verbose than I would like.

ArgumentCaptor<String> argument = ArgumentCaptor.forClass(String.class);

verify(errors, atLeastOnce()).add(argument.capture(), any(ActionMessage.class));

List<String> values = argument.getAllValues();

assertTrue(values.contains("exception.message"));
assertTrue(values.contains("exception.detail"));

First, you can't prevent people from messing with your game by declaring things 'final'. There are reflection tricks and whole code generation libraries that make it merely slightly inconvenient to get past this.

So, lose the finals, and then you can use jmock or whatever library you like to make mocks.

To answer your question directly, you cannot use Mockito to stub a method called from the constructor. Mockito needs an instance of the class before you can begin mocking, and you haven't given yourself a way to create an instance for testing.

More generally, as mentioned in Effective Java item 17, you should not call overridable methods from constructors. If you do so, for instance, you could provide an override in a subclass that refers to a final field but that runs before the final field is set. It probably won't get you in trouble here, but it's a bad habit in Java.

Luckily, you can restructure your code to do this very easily:

public class MyClass {
  public MyClass() {
    this(true);
  }

  /** For testing. */
  MyClass(boolean runSetup) {
    if (runSetup) {
      setup();
    }
  }

  /* ... */
}

To make it even more obvious, you can make the one-parameter MyClass constructor private, and provide a public static factory method:

/* ... */
  public static MyClass createForTesting() {
    return new MyClass(false);
  }

  private MyClass(boolean runSetup) {
/* ... */

Though some developers think it is a bad practice to write any code in methods that is used mostly for tests, remember that you are in charge of the design of your code, and tests are one of few consumers you absolutely know you will need to accommodate. Though it's still a good idea to avoid explicit test setup in "production" code, creating extra methods or overloads for the sake of testing will usually make your code cleaner overall and can drastically improve your test coverage and readability.

You can use a mock object to verify that you have called it in the way expected. In Mockito, mocked objects are automatically stubs, and verification occurs explicitly.

From Mockito's "Why do we need another mocking framework?":

 Separation of stubbing and verification. Should let me code in line with intuition: 
 stub before execution, selectively verify interactions afterwards. I don’t 
 want any verification-related code before execution.

You can stub the behavior of calls before they're called. For example (from the Mockito home page):

 when( mockedList.get(0)).thenReturn( "first" );

You can verify interactions with mocked objects after they're called. For example:

 verify( mockedList ).add("one");

To work around this apparent limitation, I refactored my code, replacing inheritance with delegation, and I think I've ended up with a better design anyhow since the inheritance wasn't really necessary.

The new code looks like this. Mind you the code for the question was simplified, so the real classes have much more functionality.

class S {
   S() {
      // Format user's hard drive, call 911, and initiate self-destruct
   }
}

class T {
   T(S s) {} // Now T "has an S" instead of "is an S"
}

class Test {
   @Mock private S mockS;
   new T(s); // T's call to super() should call the mock, not the destructive S.
}

For those interested, using Guice and Android, the test looks more like this:

class T {
   T(Activity activity, S s) {}
}

class Test {
  @Mock Activity activity;
  @Mock S mockS;
  injector = Guice.createInjector(new AbstractModule() {
     @Override protected void configure() {
        bind(Activity.class).toInstance(activity);
        bind(S.class).toInstance(mockS);
     }}
  );
  T t = injector.getInstance(T.class);
}

Take a look at the Mockito API docs. As the linked document mentions (Point # 12) you can use any of the doThrow(),doAnswer(),doNothing(),doReturn() family of methods from Mockito framework to mock void methods.

For example,

Mockito.doThrow(new Exception()).when(instance).methodName();

or if you want to combine it with follow-up behavior,

Mockito.doThrow(new Exception()).doNothing().when(instance).methodName();

Presuming that you are looking at mocking the setter setState(String s) in the class World below is the code uses doAnswer method to mock the setState.

World mockWorld = mock(World.class); 
doAnswer(new Answer<Void>() {
    public Void answer(InvocationOnMock invocation) {
      Object[] args = invocation.getArguments();
      System.out.println("called with arguments: " + Arrays.toString(args));
      return null;
    }
}).when(mockWorld).setState(anyString());

Several of the methods on AbstractHttpClient are final and thus won't be mocked. IMO, this behavior is the #1 reason not to mock concrete classes.