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Groups > comp.lang.java.programmer > #2908

Re: calling own methods from constructor

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On 4/6/2011 6:06 PM, Tom Anderson wrote:
> On Wed, 6 Apr 2011, Andreas Leitgeb wrote:
>
>> Is there any *good* use of having the constructor call a method that
>> actually *can* be overridden in a subclass? I mean, are there
>> (non-anti)patterns of explicitly allowing subclasses to hook into
>> base-class's construction?
>
> public abstract class Library {
> private List<Document> documents;
>
> protected Library() {
> documents = new ArrayList<Document>();
> Collection<String> titles = listDocuments();
> for (String title: titles) {
> Document doc = loadDocument(title);
> // do other preparatory stuff with the document
> documents.add(doc);
> }
> }
>
> protected abstract Collection<String> listDocuments();
> protected abstract Document loadDocument(String title);
> }
>
> public class FilesystemLibrary extends Library {
> // ...
> }
>
> public class WebDavLibrary extends Library {
> // ...
> }
>
> public class JCRLibrary extends Library {
> // ...
> }
>
> What are the alternatives?

     Safer ones, I hope.  This code presupposes that the subclass
instance can do useful work before its constructor finishes -- to
put it another way, it assumes that the subclass constructor does
absolutely nothing except call the superclass constructor (and even
that much requires some leaps of faith).

     How might Java ensure such an assumption?  I can't imagine how
it could be done at compile time, when the suite of subclasses may
not even exist to be inspected.  At run time, I guess it could be
done with two additional bits per instance: One that says "The
constructor has not yet returned" and another that says "A virtual
method has been called while the constructor is active."  Each
virtual method would copy the first bit to the second, and if the
constructor found the second bit set while doing anything other
than a "return," it could throw an exception.

> The obvious one is for the subclass constructor to prepare all the
> objects and pass them upward; i think that is likely to lead to a lot of
> duplication of effort.
>
> The almost as obvious one is to push the abstract methods out into a
> separate interface - DocumentStore, say - and have the subclass
> constructor pass up an instance of that.
>
> You could also push the repeated logic out into some sort of factory or
> helper, and have the subclasses call that, rather than relying on code
> in the supeclass, but that is repetitive, and does nothing to establish
> invariants in the superclass.

     It seems to me the constructor is doing too much of the heavy
lifting.  A Library(Collection<Document>) constructor, with the
Documents already loaded or maybe with "load on first use" flags,
seems a more tenable approach.  In particular, it allows the
subclass constructors to choose their own sets of exceptions to
throw, instead of requiring that they all extend exceptions thrown
by the superclass' abstract method declarations.

-- 
Eric Sosman
esosman@ieee-dot-org.invalid

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calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-06 20:48 +0000
  Re: calling own methods from constructor Tom Anderson <twic@urchin.earth.li> - 2011-04-06 23:06 +0100
    Re: calling own methods from constructor Lew <noone@lewscanon.com> - 2011-04-06 18:24 -0400
      Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 08:44 +0000
        Re: calling own methods from constructor Robert Klemme <shortcutter@googlemail.com> - 2011-04-07 04:14 -0700
          Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 12:32 +0000
            Re: calling own methods from constructor Robert Klemme <shortcutter@googlemail.com> - 2011-04-07 05:55 -0700
            Re: calling own methods from constructor Lew <lew@lewscanon.com> - 2011-04-07 08:10 -0700
              Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 20:31 +0000
                Re: calling own methods from constructor Lew <lew@lewscanon.com> - 2011-04-07 14:10 -0700
                Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 22:15 +0000
                Re: calling own methods from constructor Lew <noone@lewscanon.com> - 2011-04-07 21:22 -0400
              Re: calling own methods from constructor Tom Anderson <twic@urchin.earth.li> - 2011-04-07 23:08 +0100
                Re: calling own methods from constructor Lew <noone@lewscanon.com> - 2011-04-07 21:24 -0400
                Re: calling own methods from constructor "Mike Schilling" <mscottschilling@hotmail.com> - 2011-04-09 22:06 -0700
            Re: calling own methods from constructor Tom Anderson <twic@urchin.earth.li> - 2011-04-07 23:10 +0100
              Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 22:24 +0000
        Re: calling own methods from constructor Lew <noone@lewscanon.com> - 2011-04-07 07:19 -0400
          Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 12:24 +0000
    Re: calling own methods from constructor Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-06 20:46 -0400
    Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 08:36 +0000
      Re: calling own methods from constructor Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-04-07 07:16 -0300
        Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 11:26 +0000
      Re: calling own methods from constructor Paul Cager <paul.cager@googlemail.com> - 2011-04-07 03:20 -0700
        Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 11:36 +0000
  Re: calling own methods from constructor Owen Jacobson <angrybaldguy@gmail.com> - 2011-04-06 22:23 -0400
    Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 09:13 +0000
      Re: calling own methods from constructor Tobias Blass <tobiasblass@gmx.net> - 2011-04-07 10:58 +0000
        Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 12:19 +0000
          Re: calling own methods from constructor Alessio Stalla <alessiostalla@gmail.com> - 2011-04-07 10:40 -0700
            Re: calling own methods from constructor Tobias Blass <tobiasblass@gmx.net> - 2011-04-07 19:00 +0000
              Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 20:11 +0000
              Re: calling own methods from constructor Jim Janney <jjanney@shell.xmission.com> - 2011-04-07 14:36 -0600
              Re: calling own methods from constructor Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-07 20:59 -0400
    Re: calling own methods from constructor Jim Janney <jjanney@shell.xmission.com> - 2011-04-07 10:26 -0600
      Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 20:16 +0000
      Re: calling own methods from constructor Lew <lew@lewscanon.com> - 2011-04-07 14:05 -0700
        Re: calling own methods from constructor Jim Janney <jjanney@shell.xmission.com> - 2011-04-07 16:15 -0600
          Re: calling own methods from constructor Lew <noone@lewscanon.com> - 2011-04-07 21:28 -0400
          Re: calling own methods from constructor Owen Jacobson <angrybaldguy@gmail.com> - 2011-04-07 23:21 -0400
            Re: calling own methods from constructor Owen Jacobson <angrybaldguy@gmail.com> - 2011-04-07 23:39 -0400
            Re: calling own methods from constructor Owen Jacobson <angrybaldguy@gmail.com> - 2011-04-07 23:58 -0400
            Re: calling own methods from constructor Tom Anderson <twic@urchin.earth.li> - 2011-04-08 20:32 +0100
      Re: calling own methods from constructor Tom Anderson <twic@urchin.earth.li> - 2011-04-07 23:05 +0100
        Re: calling own methods from constructor Jim Janney <jjanney@shell.xmission.com> - 2011-04-07 17:38 -0600
  Re: calling own methods from constructor Roedy Green <see_website@mindprod.com.invalid> - 2011-04-07 03:20 -0700
    Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-07 11:32 +0000
  Re: calling own methods from constructor Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-04-08 01:51 +0200
    Re: calling own methods from constructor Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-04-08 01:56 +0200
      Re: calling own methods from constructor Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-04-08 09:30 +0000
        Re: calling own methods from constructor Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-04-08 20:02 +0200

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