Explanation
Using the private modifier is the main way that an object encapsulates itself and hide data from the outside world.
References:

An array variable (here xx) can very well have the null value.
Note:
Null is the reserved constant used in Java to represent a void reference i.e a pointer to
nothing. Internally it is just a binary 0, but in the high level Java language, it is a magic
constant, quite distinct from zero, that internally could have any representation.