Why Mockito's mock returns 0 when it should return null?

Question

When there's some object with a boxed type property, the getter of the property returns 0. But this should return null, because the default value of the boxed type property is null. What is the problem here?

class Person {
    private Long id;

    public Long getId() {
        return id;
    }
}
...

@Mock Person person;

...
person.getId(); // 0 instead of null

Answer 1

That's simply the chosen default value for primitive and wrapper types in the default Mockito answer.