Mocked method returns actual value instead of the mocked one

I've been asked to set up continuous integration on Android Studio so I started with unit testing and mocking.

I feel I missunderstood the Mockito basis and working. I've found this kind of examples on the Internet:

    @RunWith(MockitoJUnitRunner.class)
    public class ExampleUnitTest {

      MainActivity classToTest = new MainActivity();

      @Mock
      ClassIDontWantToTest classToMock;

      /**
       * Examples found a lot of times on the Internet
       * My method is not tested at all
       * I think it doesn't make sense to test Mockito but our own methods
       * @throws Exception
       */
      @Test
      public void testMethodIDontWantToTest() throws Exception {
        when(classToMock.methodIDontWantToTest()).thenReturn("Mocked result");
        assertTrue(classToMock.methodIDontWantToTest().equals("Mocked result"));
      }

    }

This is my code:

    public class MainActivity extends AppCompatActivity {
      ClassIDontWantToTest classIWantToMock = new ClassIDontWantToTest();

      /**
       * Actual Method I want to test
       * @return Return value
       */
      public String methodIWantToTestButReturnsUnspectedValue() {

          //  (...) Code to test

          return classIWantToMock.methodIDontWantToTest();
      }
    }


    public class ClassIDontWantToTest {

      public String methodIDontWantToTest() {
        return "Actual result";
      }

    }

And this is how I understand it would have to be done:

    @RunWith(MockitoJUnitRunner.class)
    public class ExampleUnitTest {
      MainActivity classToTest = new MainActivity();

      @Mock
      ClassIDontWantToTest classToMock;

      /**
       * Test I want to do and I think it makes sense, but it doesn't work
       * @throws Exception
       */
      @Test
      public void testMethodIWantToTest() throws Exception {
        when(classToMock.methodIDontWantToTest()).thenReturn("Mocked result");
        assertTrue(classToTest.methodIWantToTestButReturnsUnspectedValue().equals("Mocked result"));
      }
    }

Obviously there's something I don't understand, but I don't know what it is. May anybody please tell me where I'm wrong?

Thank you very much!