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Groups > comp.lang.java.programmer > #24782

Re: Interface with implied Constructor

From Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups comp.lang.java.programmer
Subject Re: Interface with implied Constructor
Date 2013-07-20 17:12 -0400
Organization A noiseless patient Spider
Message-ID <kseu52$218$1@dont-email.me> (permalink)
References (4 earlier) <Qo1Gt.8876$2C2.1824@fx04.iad> <n45ju8laf3gcaqtlerq776rti1jtid53j9@4ax.com> <kscaks$4qd$4@dont-email.me> <b500nlFgqq8U1@mid.individual.net> <kseneh$g77$1@dont-email.me>

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On 7/20/2013 3:12 PM, Martin Gregorie wrote:
> On Sat, 20 Jul 2013 19:46:16 +0200, Robert Klemme wrote:
>
>> On 19.07.2013 23:21, Martin Gregorie wrote:
>>> [...]
>>> I take the opposite approach to Robert over that for one reason: that
>>> the only way to report init errors in a constructor is to throw an
>>> Exception,
>>> which means the instance declaration often needs to go inside a
>>> try/catch block which can screw up scoping.
>>
>> In what ways does this screw up scoping?
>>
> Just that I often find that I need access to the instance outside the try/
> catch block containing the constructor call and I don't much like
> initialising the pointer as null.

     Then ... don't initialize it.

	Thing thing;
	try {
	    thing = new Thing();
	} catch (DooWopDooWopDooWopException ex) {
	    logger.log(Level.SEVERE, "Eek!", ex);
	    throw ex;
	}
	System.out.println(thing.getSwing());
	...

This form has the advantage that the compiler will complain if
the all-important `throw ex;' is forgotten, whereas initializing
to null would just get you an NPE.

     Very occasionally there's a situation where you really do need
to use the null-ness of `thing' as an indication that construction
failed.  In that case, I'd still suggest not initializing:

	Thing thing;
	try {
	    thing = new Thing();
	} catch (DooWopDooWopDooWopException ex) {
	    logger.log(Level.SEVERE, "Eek!", ex);
	    thing = null;
	}
	if (thing != null) {
	    System.out.println(thing.getSwing());
	else {
	    System.out.println("It don't mean a thing");
	}

IMHO, situations like this are suggestive of a poorly-designed
Thing class (which you may be forced to endure), or of a method
that's trying to do too many things and might better be split up.

> This is a result of my dislike of using Exceptions to signal anything
> except a fatal error and may well be a hangover from writing a lot of C
> in the past. A method that returns reference to a library class whose
> constructor can throw an exception, e.g. Integer, but does not itself
> return an exception requires this sort of structure:
>
> public class Altitude
> {
>    int    alt   = 0;
>    String error = null;
>
>    /**
>     * Returning a negative number indicates an invalid height.
>     */
>    public int getHeight(String height)
>    {
>      Integer h = null;

     Since this initialization is useless (the variable will
always be overwritten, exception or no), why even have it?

>      try
>      {
>        h = new Integer(height);
>        if (h < 0)
>          error = "Below MSL";
>      }
>      catch (NumberFormatException e)
>      {
>        error = e.getMessage();
>        h = new Integer(-1);
>      }
>
>      return alt = h.intValue();
>    }
>
>    public boolean isValid()
>    { return error === null; }
>
>    public String getError()
>    { return error; }
> }
>
> This isn't a really good example because Integer provides other ways of
> doing the job, but it does illustrate the sort of scoping problem I'm
> talking about: that of allowing access to the Integer instance in both
> parts of the try/catch block as well as from surrounding parts of the
> method body.

     I don't see how the NumberFormatException has much to do
with it: Your getHeight() method needs the `h' at the point
of its return, so its scope must include the return statement.
The only "scoping issue" I can see is that the try block and the
catch block have their own subsidiary scopes in which `h' also
needs to be (and is) visible -- but then, why aren't you worried
about the scope of `height'?  (Or even of `error' and `alt'?)

-- 
Eric Sosman
esosman@comcast-dot-net.invalid

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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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