Re: What's the use of private?
Jeff.M wrote:
I understand the composition is preferable to inheritance, but that's
not what I'm asking about.
I'm afraid I don't see how private members makes extending easier.
Let's say you wanted to extend some LinkedList class, or whatever, and
the instance variable for the head node is private. Then you couldn't
do anything with it, and you in all probably would find it impossible
to write the code you need to write.
Classes have to be designed for inheritance. If the 'head' should be
inherited, then it should be declared private. OTOH, if there are special
checks to be made for 'head', such as making sure that it isn't null at the
wrong time, or that it holds the right pair of pointers, or whaterver, then
making the variable protected risks breaking program invariants. In that case
it is better to expose 'head' as a property (via protected getters and
setters) and keep the variable itself private. Thus subclasses are relieved
of a portion of the implementation, and encapsulation is preserved.
Protected also supports the principle of encapsulation and information
hiding. Any code that uses a class (rather than extends) will not be
able to access protected members. And so I don't think I'd agree that
protected members are part of a class's outer face.
Protected access supports encapsulation less than private. The problem with
protected access is that anyone can create a subclass and futz with the
protected thing. Protected does not support encapsulation - the bad premise
leads to bad conclusions. Protected opens up an element, compared to private
or package-private, *breaking* encapsulation.
--
Lew