How to use informs_about_unused_stubs_due_arg_mismatch method of org.mockito.internal.debugging.WarningsFinderTest class

Actually org.mockito.Matchers.eq uses org.mockito.internal.matchers.Equality.areEqual(Object o1, Object o2) method to check equality of given matcher to actual value passed to the method. What is interesting, the implementation of this method was stolen from hamcrest as is stated in comment:

//stolen from hamcrest because I didn't want to have more dependency than Matcher class 
//...
public static boolean areEqual(Object o1, Object o2) {
    if (o1 == o2 ) {
        return true;
} else if (o1 == null || o2 == null) {
        return o1 == null && o2 == null;
    } else if (isArray(o1)) {
        return isArray(o2) && areArraysEqual(o1, o2);
    } else {
        return o1.equals(o2);
    }
}

Place a breakpoint at the very beginning of this method to see what is happening in your own code. Since you pass ArrayList to the argument of eq() you may need to dig deeper into areArraysEqual and areArrayLengthsEqual methods.

Some of the other answers offer alternatives to doing exactly what I want here, but this is doable by Spying the Consumer class itself and having the spy call the method you really want to execute. A helper method for wrapping the lambda to create the spy helps here:

/** Get a spied version of the given Consumer. */
private Consumer<Item> itemHandlerSpy(Consumer<Item> itemHandler) {
  // Create a spy of the Consumer functional interface itself.
  @SuppressWarnings("unchecked")
  Consumer<Item> spy = (Consumer<Item>) Mockito.spy(Consumer.class);
  // Tell the spy to run the given consumer when the Consumer is given something to consume. 
  Mockito.doAnswer(it -> {
    // Get the first (and only) argument passed to the Consumer.
    Item item = (Item) it.getArguments()[0];
    // Pass it to the real consumer so it gets processed.
    itemHandler.accept(item);
    return null;
  }).when(spy).accept(Mockito.any(Item.class));
  return spy;
}

And then the test method becomes very straightforward:

Consumer<Item> itemHandler = itemHandlerSpy(Item::foo);
instance.conditionalRun(itemHandler);
// This verifies conditionalRun called the Consumer exactly once.
Mockito.verify(itemHandler).accept(Mockito.any(Item.class));

You should avoid creating business objects with new in a Spring application. You should always get them from the application context - it will ease mocking them in your test.

In your use case, you should simply create your validator as a bean (say defaultTerminationValidator) and inject it in your controller :

public class TerminationController extends AbstractController {

    private TerminationValidator terminationValidator;

    @Autowired
    public setDefaultTerminationValidator(TerminationValidator validator) {
        this.terminationValidator = validator;
    }

    @InitBinder("termination")
    public void initBinder(WebDataBinder binder, HttpServletRequest request) {
        binder.setValidator(terminationValidator);
        binder.setAllowedFields(new String[] { "termId[**]", "terminationDate",
                "accountSelection", "iban", "bic" });
    }

    [...]
}

That way, you will be able to simply inject a mock in your test.

I have to strongly disagree with this question.

There is no justification for a mocking tool that limits design choices. It's not just static methods that are ruled out by EasyMock, EasyMock Class Extension, jMock, Mockito, and others. These tools also prevent you from declaring classes and methods final, and that alone is a very bad thing. (If you need one authoritative source that defends the use of final for classes and methods, see the "Effective Java" book, or watch this presentation from the author.)

And "initialising collaborators rather than injecting them" often is the best design, in my experience. If you decompose a class that solves some complex problem by creating helper classes that are instantiated from that class, you can take advantage of the ability to safely pass specific data to those child objects, while at the same time hiding them from client code (which provided the full data used in the high-level operation). Exposing such helper classes in the public API violates the principle of information hiding, breaking encapsulation and increasing the complexity of client code.

The abuse of DI leads to stateless objects which really should be stateful because they will almost always operate on data that is specific to the business operation. This is not only true for non-public helper classes, but also for public "business service" classes called from UI/presentation objects. Such service classes are usually internal code (to a single business application) that is inherently not reusable and have only a few clients (often only one) because such code is by nature domain/use-case specific. In such a case (a very common one, by the way) it makes much more sense to have the UI class directly instantiate the business service class, passing data provided by the user through a constructor.

Being able to easily write unit tests for code like this is precisely what led me to create the JMockit toolkit. I wasn't thinking about legacy code, but about simplicity and economy of design. The results I achieved so far convinced me that testability really is a function of two variables: the maintainability of production code, and the limitations of the mocking tool used to test that code. So, if you remove all limitations from the mocking tool, what do you get?

You would need a spy, not a mock A:

    A a = Mockito.spy(new A(1,1));
    a.getPrice(2);
    verify(a, times(1)).getPriceForOne();

After a lot of painful Maven trial & error I struggled across this website, which provides a surprisingly easy way to use unit-tests in a Maven-Tycho setup.

Here, the important parts of the pom.xml when using JUnit (probably looks similar for Mockito):

<testSourceDirectory>src/test/java</testSourceDirectory>
<dependencies>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.11</version>
        <scope>test</scope>
    </dependency>
</dependencies>
<plugins>
  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <version>2.12.4</version>
    <executions>
      <execution>
        <id>test</id>
        <phase>test</phase>
        <configuration>
          <includes>
            <include>**/*Test.java</include>
          </includes>
        </configuration>
        <goals>
          <goal>test</goal>
        </goals>
      </execution>
    </executions>
  </plugin>

  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>2.5.1</version>
    <executions>
      <execution>
        <id>compiletests</id>
        <phase>test-compile</phase>
        <goals>
          <goal>testCompile</goal>
        </goals>
      </execution>
    </executions>
  </plugin>
</plugins>

Name all your tests in a way so that they end with *Test.java. Run mvn test in order to execute all available unit-tests.

Since Mockito 1.9.5+ and Java 8+

You can use a lambda expression, like:

when(myMock.myFunction(anyString())).thenAnswer(i -> i.getArguments()[0]);

Where i is an instance of InvocationOnMock.

For older versions

You can create an Answer in Mockito. Let's assume, we have an interface named MyInterface with a method myFunction.

public interface MyInterface {
    public String myFunction(String abc);
}

Here is the test method with a Mockito answer:

public void testMyFunction() throws Exception {
    MyInterface mock = mock(MyInterface.class);
    when(mock.myFunction(anyString())).thenAnswer(new Answer<String>() {
    @Override
    public String answer(InvocationOnMock invocation) throws Throwable {
        Object[] args = invocation.getArguments();
        return (String) args[0];
    }
    });

    assertEquals("someString",mock.myFunction("someString"));
    assertEquals("anotherString",mock.myFunction("anotherString"));
}

Let's first separate interfaces from classes.

Interfaces are great for mocking (thus allowing for testing your app without the need for the sluggish GWTTestCase):

• IsWidget: when all you need is a handle on a widget, without depending on the Widget class. This is typically used with MVP as a way to represent the view.
• AcceptsOneWidget: when you need a placeholder for a single widget (in the form of an IsWidget). This is typically used with Activities, to insert the view (IsWidget) into the given slot (AcceptsOneWidget).

The classes you list all extend Widget, so they rely on JSNI and (most of the time) need to run in a GWT environment (for unit tests, that means a GWTTestCase):

• Widget: the base of all widgets. Implements IsWidget returning itself from asWidget().
• Composite: a base class when you need to create a widget built from other widgets while hiding their implementation. While you could extend an existing widget, it's generally better to hide it inside a Composite so you only expose the API you need/want to expose. Composite is about "composition rather than inheritance" and encapsulation. Examples of composites in standard widgets include TabPanel (built from a TabBar and DeckPanel), DateBox (built from a TextBox and DatePicker in a PopupPanel), ValueListBox that wraps a ListBox or ValuePicker that wraps a CellList. In many cases, given that panels accept IsWidget children, you could simply implement IsWidget rather extend Composite, but it's sometimes useful to have a true Widget.
• SimplePanel a panel that implements AcceptsOneWidget, useful as a slot when using activities (but you could also easily implement AcceptsOneWidget to insert into any kind of panel)

That being said, Google recently open-sourced GWT-Mockito that plugs Mockito into GWT.create() and uses classloader magic to rewrite JSNI methods and remove final modifiers so you can directly use widgets in tests without the need for GWTTestCase or MVP.

So, all in all, it depends how you approach your code, how you architecture your app. If you use MVP, stick to depending on interfaces only (IsWidget, AcceptsOneWidget) in your presenter so you can easily mock your view in your tests.
Otherwise, or if you want a "simplified MVP" where the view is a UiBinder template, try GWT-Mockito for your tests and directly use widgets.
Of course, you can mix both approaches in the same application. And in any case, build your own widgets as Widgets for low-level things (rarely needed), and Composites or IsWidgets for everything else, rather than extending existing widgets.

You should instantiante testRef as follows:

Test testRef = new Test(taskJob) {

    public void updateAndReschedule(Long id, String message) {
        //do nothing
    }

};

No need for the spy.