Re: contructors setters or both

From: glunk (stowe_at_whackthisvsac.org)
Date: 06/30/04 

Date: Wed, 30 Jun 2004 10:02:14 -0400

"Chris Smith" <cdsmith@twu.net> wrote in message
news:MPG.1b4c72da36907cce98969f@news.altopia.net...
> glunk wrote:
> > I went to a Java class (training!) a while ago. The instructor was
actually
> > a real programmer. I am used to Microsoft training in which the person
doing
> > the training does not actually ever DO the work. Anyway, the instructor
told
> > us it was a good idea to either set the values of instance variables by
> > passing them into the constructor OR by setter methods, but not both.
>
> I'm not sure exactly what your instructor was referring to here. What
> he/she *may* have been talking about is that there are occasions where
> you ought to consider writing an immutable class, in which there is no
> way to change the data stored in an object after it has been created.
> Examples from the core API include java.lang.String and
> java.lang.Integer.
>
> Immutable classes have a number of advantages, including that they can
> be safely shared between threads and between pieces of the program,
> without worrying that one piece of code will change the data in the
> object and interfere with other code that is also using the same object.
>
> However, once you've decided to make an object mutable (i.e., it can be
> changed), there's no reason not to also provide a constructor that lets
> you set an initial value. There's also no requirement that if you write
> such mutator methods, that you use them from inside the class (although
> you certainly can; see below). Mutators are quite convenient for
> external use as well. For example, it's easier to say:
>
> return new Appointment("Dr. Johnson", apptTime);
>
> versus:
>
> Appointment appt = new Appointment();
> appt.setDoctor("Dr. Johnson");
> appt.setTime(apptTime);
> return appt;
>
> > We were going to do this kind of thing
> >
> >
> > public MyClass (String lastName, String firstName)
> > {
> > setLastName(lastName);
> > setFirstName(firstName);
> > }
> >
> > to ensure that any setter code which may do something to the value is
used.
> >
> > But then, if you can see my other post entitled "getting used to Java -
> > question about 'style'" this seems to be calling a nonfinal method (the
> > setters) from within a constructor.
>
> Yes, it is, assuming that the class is not final and that the mutator
> methods are not final. Are you designing this class for use of
> inheritance? If so, then you would indeed need to work this out.
>
> For the most part, mutators aren't used from inside the class where you
> use them, which would solve this problem for you. The most important
> unit of encapsulation in an OO language is the class, and it's okay to
> break encapsulation inside of a class, because you assume that you're in
> control of all that code.
>
> Of course, use of mutators even for private fields (sometimes called
> self-encapsulation) can be a nice implementation convenience to avoid
> duplicating code. In this case, one possibility might be to provide
> both private mutators (for self-encapsulation within the class) and
> public mutators (which just call the private mutators by default), and
> then call the private mutators directly from the constructor. That
> means you end up with *two* mutators for each field, though, which can
> get unwieldly rather fast.
>
> > Can someone comment on whether or not it is generally wise to not have
> > setters if the instance vars have to be set in the constructor? In this
> > example, I cannot even think of a value changing.
>
> Sounds like, in this case, you'd be better off going for an immutable
> class, and not providing any mutators at all. It really depends on your
> overall design, though, so I can't make that call for you.
>

Thanks. You are very good at explaining.

S

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