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Groups > comp.lang.java.programmer > #24795

Re: Interface with implied Constructor

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On 07/20/2013 03:09 PM, Robert Klemme wrote:
> On 19.07.2013 04:21, Arved Sandstrom wrote:
>> On 07/14/2013 01:03 PM, Robert Klemme wrote:
>>> On 10.07.2013 00:46, Richard Maher wrote:
>
>>>> So let me ask you "Why do you want to use setters or an init() method?"
>>>
>>> I opted for the init() method OR factory method pattern.  Both are type
>>> safe because in both cases a method in an interface needs to be
>>> implemented.  Picking a constructor solely based on argument types has a
>>> looser coupling between the contract and the implementation of it.  An
>>> interface makes that coupling explicit.
>
>> I'll be honest, Robert, personally I consider the behaviour of an object
>> - the kinds of things you could specify in an interface - to be what the
>> *initialized* object can (should) do.
>
> Of course.  A factory instance can also be an initialized object.

I would hope that it would usually be. But since Socket got mentioned, 
we see that that is not always the case.

But I have no aversion to factory methods.

>> Often the best way to properly initialize an object is to call a
>> constructor with arguments. That's what they are for. And to be honest
>> once again, a ctor *is* an implementation thing, IMO: it's not the
>> business of an interface to dictate how an implementation will
>> initialize objects that can support the interface contract. After all,
>> how can an interface even possibly imagine what the internal state of an
>> initialized object, hence what ctor args are required, is?
>
> In situations like Richard's (at least as I understand it) the
> environment knows what information it has available and the interface
> for the factory can be defined in such a way to pass data potentially
> needed or the data that the environment is willing to provide.  The
> factory implementation then can decide which part of that information is
> needed to create the instance and ignore the rest.  It's surely better
> to let the implementor of the construction process decide which
> information to use than the implementor of the environment (or call it
> "framework").

You know, I'd call that a 50/50, I think sometimes the caller is the 
best judge, and sometimes the callee is.

>> No problem with factory methods, except they are for specific
>> situations. And I think they also lean more towards implementation.
>
> What "specific situations" do you have in mind?  As far as I can see the
> name of the class to create is configured and Richard said reflection
> will be used in his initial posting.  I think that is a typical
> situation where a factory class can be configured which is created via a
> no arg constructor and which has a method createFoo(...) with
> appropriate arguments.  Then there is no need to fiddle with reflection
> to analyze constructors and argument lists.  Also, this gives more
> flexibility as the factory can decide which class to instantiate at
> runtime if need be.

Specific situation? The very essence of a factory method - you need to 
create an object that can do such-and-such, but you don't (for one 
reason or another), or can't, specify the exact class of the object to 
be created.

By definition, if you *do* know exactly what you want to create, and 
have appropriate constructors to do it with, why resort to a factory method?

> My main point is that I dislike using reflection to _find_ an
> appropriate method - be it a constructor or another method.  I prefer
> that method to be identified in an interface - whether that is an init()
> method in the interface of the object I will eventually be using or a
> create() method in another interface defining the factory API I care
> less.  I think Class.forName() and newInstance() is enough reflection to
> get flexibility and decoupling but from there on I prefer to use methods
> specified via an interface or abstract base class.
>
> Kind regards
>
>      robert
>
I'd differentiate between the use of reflection to find a method 
(including ctors) and the more general discussion we strayed into, which 
is simply when do you use ctors or factory methods or whatever.

In the last 5 years I think I've extensively used reflection in Java 
_twice_. In one case, as I mentioned previously in this thread, it was 
for construction of non-JPA entity result objects for native queries; I 
adapted a third party class to modify and adapt how it searched for 
constructors, but otherwise it was a reasonable use...compared to the 
alternative.

In the other case I had to write a "command server" because of the 
erstwhile fork+exec problem on Solaris, where RAM would get hammered. 
This server basically took on the job of executing system programs 
through Runtime and Process, running in a much smaller heap than the 
calling apps. Because of it's very nature, it had to load or unload 
command classes at runtime, and these classes did have to be written to 
an interface. No need for specifying how they constructed themselves though.

AHS

-- 
When a true genius appears, you can know him by this sign:
that all the dunces are in a confederacy against him.
-- Jonathan Swift

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Thread

Interface with implied Constructor Richard Maher <maher_rjSPAMLESS@hotmail.com> - 2013-07-09 18:41 +0800
  Re: Interface with implied Constructor Robert Klemme <shortcutter@googlemail.com> - 2013-07-09 04:08 -0700
    Re: Interface with implied Constructor Richard Maher <maher_rjSPAMLESS@hotmail.com> - 2013-07-10 06:46 +0800
      Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-09 21:08 -0400
      Re: Interface with implied Constructor Kevin McMurtrie <mcmurtrie@pixelmemory.us> - 2013-07-11 19:11 -0700
        Re: Interface with implied Constructor Robert Klemme <shortcutter@googlemail.com> - 2013-07-14 17:53 +0200
          Re: Interface with implied Constructor Kevin McMurtrie <mcmurtrie@pixelmemory.us> - 2013-07-15 08:58 -0700
            Re: Interface with implied Constructor Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-07-15 10:57 -0700
              [OT] Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-15 14:13 -0400
                Re: [OT] Re: Interface with implied Constructor Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-07-15 12:58 -0700
            Re: Interface with implied Constructor Robert Klemme <shortcutter@googlemail.com> - 2013-07-15 20:45 +0200
      Re: Interface with implied Constructor Robert Klemme <shortcutter@googlemail.com> - 2013-07-14 18:03 +0200
        Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-18 23:21 -0300
          Re: Interface with implied Constructor Gene Wirchenko <genew@telus.net> - 2013-07-19 12:38 -0700
            Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-19 17:56 -0300
              Re: Interface with implied Constructor Gene Wirchenko <genew@telus.net> - 2013-07-22 12:39 -0700
                Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-22 16:02 -0400
                Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-22 18:08 -0300
                Re: Interface with implied Constructor Gene Wirchenko <genew@telus.net> - 2013-07-23 14:32 -0700
                Re: Interface with implied Constructor Jim Gibson <jimsgibson@gmail.com> - 2013-07-23 14:43 -0700
                Re: Interface with implied Constructor Gene Wirchenko <genew@telus.net> - 2013-07-24 13:27 -0700
                Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-24 18:20 -0300
                Re: Interface with implied Constructor Robert Klemme <shortcutter@googlemail.com> - 2013-07-26 00:03 +0200
                Re: Interface with implied Constructor Martin Gregorie <martin@address-in-sig.invalid> - 2013-07-24 21:18 +0000
                Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-24 17:42 -0400
                Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-23 18:19 -0400
                Re: Interface with implied Constructor Gene Wirchenko <genew@telus.net> - 2013-07-24 13:31 -0700
                Re: Interface with implied Constructor Steven Simpson <ss@domain.invalid> - 2013-07-27 04:06 +0100
            Re: Interface with implied Constructor Martin Gregorie <martin@address-in-sig.invalid> - 2013-07-19 21:21 +0000
              Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-19 18:56 -0300
                Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-19 18:10 -0400
                Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-19 20:46 -0300
                Re: Interface with implied Constructor Richard Maher <maher_rjSPAMLESS@hotmail.com> - 2013-07-20 12:42 +0800
                Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-20 09:08 -0300
                Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-20 09:08 -0400
              Re: Interface with implied Constructor Kevin McMurtrie <mcmurtrie@pixelmemory.us> - 2013-07-19 19:20 -0700
                Re: Interface with implied Constructor markspace <markspace@nospam.nospam> - 2013-07-19 20:11 -0700
                Re: Interface with implied Constructor Richard Maher <maher_rjSPAMLESS@hotmail.com> - 2013-07-20 12:26 +0800
                Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-20 09:38 -0300
              Re: Interface with implied Constructor Robert Klemme <shortcutter@googlemail.com> - 2013-07-20 19:46 +0200
                Re: Interface with implied Constructor Martin Gregorie <martin@address-in-sig.invalid> - 2013-07-20 19:12 +0000
                Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-20 17:12 -0400
                Re: Interface with implied Constructor Martin Gregorie <martin@address-in-sig.invalid> - 2013-07-20 21:18 +0000
                Re: Interface with implied Constructor Robert Klemme <shortcutter@googlemail.com> - 2013-07-21 12:13 +0200
                Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-21 16:18 -0300
                Re: Interface with implied Constructor markspace <markspace@nospam.nospam> - 2013-07-21 13:45 -0700
                Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-21 16:58 -0400
                Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-21 21:19 -0300
          Re: Interface with implied Constructor Robert Klemme <shortcutter@googlemail.com> - 2013-07-20 20:09 +0200
            Re: Interface with implied Constructor Robert Klemme <shortcutter@googlemail.com> - 2013-07-20 20:21 +0200
            Re: Interface with implied Constructor Joerg Meier <joergmmeier@arcor.de> - 2013-07-20 23:17 +0200
              Re: Interface with implied Constructor Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-20 17:32 -0400
                Re: Interface with implied Constructor Joerg Meier <joergmmeier@arcor.de> - 2013-07-21 00:10 +0200
            Re: Interface with implied Constructor Arved Sandstrom <asandstrom2@eastlink.ca> - 2013-07-21 17:53 -0300
  Re: Interface with implied Constructor markspace <markspace@nospam.nospam> - 2013-07-09 18:13 -0700
    Re: Interface with implied Constructor Richard Maher <maher_rjSPAMLESS@hotmail.com> - 2013-07-10 18:03 +0800
      Re: Interface with implied Constructor markspace <markspace@nospam.nospam> - 2013-07-10 08:41 -0700
        Re: Interface with implied Constructor Richard Maher <maher_rjSPAMLESS@hotmail.com> - 2013-07-11 06:53 +0800
  Re: Interface with implied Constructor Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-07-09 21:41 -0700
    Re: Interface with implied Constructor Richard Maher <maher_rjSPAMLESS@hotmail.com> - 2013-07-10 17:55 +0800
    Re: Interface with implied Constructor Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-07-10 14:52 -0700
  Re: Interface with implied Constructor Roedy Green <see_website@mindprod.com.invalid> - 2013-07-10 16:02 -0700

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