@SpyBean vs @MockBean /spies vs mocks in programmatic testing

MockAndSpy.java

/*
@MockBean //or Mockito's @Mock
- it mocks the object and all its methods with do nothing and their result value will be null,
- use for example: when(...) methods to create mocked method behaviour
- use when you want to completely get rid of the object's normal behaviour
- caution: unmocked methods won't work -> can result in a RuntimeException during tests (@SpyBean is a remedy here apart from wider mocking)
@SpyBean //or Mockito's @Spy
- an object will behave like an @Autowired object
- all its methods will actually works, but we can define some custom behavior for its methods 
- use doReturn(...) / doNothing(...) to add custom (mocked) method behaviour 
- use if you want to provide your mock behaviour but not dismiss entirely its normal behaviour
*/
@Mock
private List<String> mockList;

@Spy
private List<String> spyList = new ArrayList();

@Test
public void testMockList() {
    //by default, calling the methods of mock object will do nothing
    mockList.add("test");
    assertNull(mockList.get(0));
}

@Test
public void testSpyList() {
    //spy object will call the real method when not stub
    spyList.add("test");
    assertEquals("test", spyList.get(0));
}

@Test
public void testMockWithStub() {
    //try stubbing a method
    String expected = "Mock 100";
    when(mockList.get(100)).thenReturn(expected);

    assertEquals(expected, mockList.get(100));
}

@Test
public void testSpyWithStub() {
    //stubbing a spy method will result the same as the mock object
    String expected = "Spy 100";
    //take note of using doReturn instead of when
    doReturn(expected).when(spyList).get(100);

    assertEquals(expected, spyList.get(100));
}

//source: https://javapointers.com/tutorial/difference-between-spy-and-mock-in-mockito/