How to use smart_nulls_generic_bug_generic_M_provided_as_varargs method of org.mockitousage.stubbing.SmartNullsGenericBugTest class

At first you should check your test logic. Usually there are 3 cases. First, you are mocking the wrong method (you made a typo or someone changed tested code so that mocked method is no longer used). Second, your test is failing before this method is called. Third, your logic falls in wrong if/switch branch somewhere in the code so that mocked method is not called.

If this is the first case you always want to change the mocked method for the one used in the code. With the second and the third it depends. Usually you should just delete this mock if it has no use. But sometimes there are certain cases in parametrized tests, which should take this different path or fail earlier. Then you can split this test into two or more separate ones but that's not always good looking. 3 test methods with possibly 3 arguments providers can make your test look unreadable. In that case for JUnit 4 you silent this exception with either

@RunWith(MockitoJUnitRunner.Silent.class) 

annotation or if you are using rule approach

@Rule
public MockitoRule rule = MockitoJUnit.rule().strictness(Strictness.LENIENT);

or (the same behaviour)

@Rule
public MockitoRule rule = MockitoJUnit.rule().silent();

For JUnit 5 tests you can silence this exception using this annotation provided in mockito-junit-jupiter package:

@ExtendWith(MockitoExtension.class)
@MockitoSettings(strictness = Strictness.LENIENT)
class JUnit5MockitoTest {
}

By annotating the BImpl field with @Mock, you're saying that the instance itself should be a mock. However, you're not telling Mockito what result the mock should give when invoked with arguments, so Mockito uses its default behaviour of returning null.

The fact that BImpl is a real class with real behaviour is irrelevant. The field does not contain an instance of that class; it contains an instance of a mock.

In your test method, before you call aImpl.doSomething(), tell mockito the behaviour you expect it to use:

when(bImpl.permuteString("test")).thenReturn("someOtherValue");

Reason: your test load one application context with bean mock inside. The bean mocks are dirty after any test. You have a dirty application context also.

Solution: you have to clean bean mock. Using Mockito.reset(mocks) after each test. I've found sth similar: How to clean up mocks in spring tests when using Mockito

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 should matcher that matches long not any object:

verify(foo, times(1)).causeProblems(anyLong());

I checked that it runs correctly.

Two more ways to do it (see my comment on the previous answer by @Tomasz Nurkiewicz):

The first relies on the fact that the compiler simply won't let you pass in something of the wrong type:

when(a.method(any(Class.class))).thenReturn(b);

You lose the exact typing (the Class<? extends A>) but it probably works as you need it to.

The second is a lot more involved but is arguably a better solution if you really want to be sure that the argument to method() is an A or a subclass of A:

when(a.method(Matchers.argThat(new ClassOrSubclassMatcher<A>(A.class)))).thenReturn(b);

Where ClassOrSubclassMatcher is an org.hamcrest.BaseMatcher defined as:

public class ClassOrSubclassMatcher<T> extends BaseMatcher<Class<T>> {

    private final Class<T> targetClass;

    public ClassOrSubclassMatcher(Class<T> targetClass) {
        this.targetClass = targetClass;
    }

    @SuppressWarnings("unchecked")
    public boolean matches(Object obj) {
        if (obj != null) {
            if (obj instanceof Class) {
                return targetClass.isAssignableFrom((Class<T>) obj);
            }
        }
        return false;
    }

    public void describeTo(Description desc) {
        desc.appendText("Matches a class or subclass");
    }       
}

Phew! I'd go with the first option until you really need to get finer control over what method() actually returns :-)

Mockito has an isNull matcher, where you can pass in the name of the class. So if you need to use it with other matchers, the correct thing to do is

verify(client).method(eq("String"),isNull(Class<?>.class));

This is now deprecated, see the answer below for the new method - https://stackoverflow.com/a/41250852/1348

I would recommend approach #1, but would use Spring for dependency injection rather than instantiating DAOs directly.

This way, for unit testing the client code, you can substitue mock DAOs, and verify that the correct DAOs are invoked with the appropriate arguments. (Mockito is useful here.)

If you use static methods, then unit testing is much more difficult, since static methods cannot be overridden.

Since class A is mocked, all method invocations wont go to the actual object. Thats why your second assert fails (i guess it might have returned 0).

Solution:

You could do something like

when(test.b()).thenCallRealMethod();

else you could spy like

A test = spy(new A());
Mockito.when(test.a()).thenReturn(35);
assertEquals(35,test.a());
assertEquals(35,test.b());