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Re: Question re testing constructor

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Ian Shef <invalid@avoiding.spam> wrote in
news:Xns9FB45B83313A1vaj4088ianshef@138.125.254.103: 

> Lew <lewbloch@gmail.com> wrote in
> news:26281492.354.1323270874867.JavaMail.geo-discussion-forums@pret21: 
> 
>> Ian Shef wrote:
>>> Discussion:  The JLS seems to treat casting separately from the
>>> operations (such as calling a method) that would follow a cast.  It
>>> "feels like" this doesn't need to be the case.  Thus I agree with
>>> Stefan that it "feels like" the compiler or an IDE could catch this at
>>> compile time (Eclipse does not) if the JLS permitted or required it. 
>>> Perhaps someone can explain why this is not the case.
>> 
>> Short answer: Because despite its best efforts, the compiler cannot
>> cure stupidity.
> 
> Agreed.
> 
>> 
>> The compiler likely sees this as not different from 
>> '((PrintStream)variable).println()', i.e., as a candidate for a 
>> 'NullPointerException' rather than a compiler error.
> 
> That would seem to be the intent of the JLS.  The compiler follows the
> rules laid down by the JLS.
> 
> On the other hand, there are many places in the JLS where constants or 
> literals are treated specially, and where responsibility is shared
> between the compiler and the JVM.  Casts are not treated this way and
> are the responsibility of the JVM (with some minor exceptions, such as
> when the compiler can guarantee that a cast is unnecessary e.g., a
> widening reference conversion).
> 
> My preference is to find such errors as early as possible.  However, the
> compiler can't save programmers from all folly.  _Java Puzzlers_ by
> Joshua Bloch and Neal Gafter has other examples where a cast is
> "obviously" an error  but it won't be caught until run time.  [My copy
> of the book is currently 25 miles away, so I can't provide a more
> precise reference.] 
> 

I now have the book in my hands (5th Printing  June 2008).  The third 
program in "Puzzle 50: Not Your Type" looks like this:

public class Type3 {
    public static void main(String args[]) {
        Type3 t3 = (Type3) new Object();
    }
}

Any typos are mine, not the authors'.

SPOILER ALERT










This program compiles but fails at run time.  The compiler (per the JLS) 
requires that Object be a subtype of Type3, or vice versa.

The authors write "Although it is obvious to us that this cast will fail, 
the type system is not powerful enough to know that the run-time type of 
the expression  new Object()  cannot be a subtype of Type3.  Therefore, the 
program throws a  ClassCastException  at run time."

The first and second programs of this puzzle involve instanceof, which has 
behavior which is closely related to that of the cast operator.  Although 
the programs are quite simple, the results are counterintuitive (but in 
accordance with the JLS).

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Question re testing constructor Novice <novice@example..com> - 2011-12-06 15:14 +0000
  Re: Question re testing constructor markspace <-@.> - 2011-12-06 07:23 -0800
    Re: Question re testing constructor Novice <novice@example..com> - 2011-12-06 17:17 +0000
    Re: Question re testing constructor Tom Anderson <twic@urchin.earth.li> - 2011-12-06 21:25 +0000
    Re: Question re testing constructor Ian Shef <invalid@avoiding.spam> - 2011-12-07 00:29 +0000
      Re: Question re testing constructor Patricia Shanahan <pats@acm.org> - 2011-12-06 19:28 -0800
      Re: Question re testing constructor Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-12-06 22:57 -0600
      Re: Question re testing constructor Lew <lewbloch@gmail.com> - 2011-12-07 07:12 -0800
      Re: Question re testing constructor Lew <lewbloch@gmail.com> - 2011-12-07 07:14 -0800
        Re: Question re testing constructor Ian Shef <invalid@avoiding.spam> - 2011-12-07 15:59 +0000
          Re: Question re testing constructor Ian Shef <invalid@avoiding.spam> - 2011-12-08 19:19 +0000
      Re: Question re testing constructor ilAn <idonot@wantspam.net> - 2011-12-07 20:41 +0200
  Re: Question re testing constructor Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-12-06 07:34 -0800
    Re: Question re testing constructor Novice <novice@example..com> - 2011-12-06 17:15 +0000
  Re: Question re testing constructor Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-06 09:49 -0800
  Re: Question re testing constructor Roedy Green <see_website@mindprod.com.invalid> - 2011-12-06 16:40 -0800
  Re: Question re testing constructor Henk van Voorthuijsen <voorth@xs4all.nl> - 2011-12-07 00:33 -0800

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