how come mock doesn't return null when no subbing defined

I looked at mockito official doc

and I don't understand:

1. Let's verify some behaviour!

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//Let's import Mockito statically so that the code looks clearer
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import static org.mockito.Mockito.*;
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//mock creation
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List mockedList = mock(List.class);
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//using mock object
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mockedList.add("one");
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mockedList.clear();
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//verification
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verify(mockedList).add("one");
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verify(mockedList).clear();
Once created, mock will remember all interactions. Then you can selectively verify whatever interaction you are interested in.

2. How about some stubbing?

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//You can mock concrete classes, not only interfaces
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LinkedList mockedList = mock(LinkedList.class);
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//stubbing
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when(mockedList.get(0)).thenReturn("first");
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when(mockedList.get(1)).thenThrow(new RuntimeException());
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//following prints "first"
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System.out.println(mockedList.get(0));
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//following throws runtime exception
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System.out.println(mockedList.get(1));
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//following prints "null" because get(999) was not stubbed
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System.out.println(mockedList.get(999));

how come

08 mockedList.add("one");

doesn't return null, as no stubbing was defined.

as seen here:

14 //following prints "null" because get(999) was not stubbed 15 System.out.println(mockedList.get(999));