I found the faulty part.
I changed
@RunWith(MockitoJUnitRunner.class)
to
@RunWith(PowerMockRunner.class)
Now the mocking works. But i have to say that as Jon Skeet printed out, the enum does not have everywhere that mocked member-instance. So in another Unit test calling MyEnum.CONSTANT.sayHello();
will print again it works
instead of Hello!
.