e==null? really?

Is it possible to do this in Java?

throw null;


What probably happened if I am looking at code like this:

} catch (Exception e) {

and e is null?

I'm seeing this, and I just don't get it.

On 7/13/2012 7:18 AM, bob smith wrote:
> Is it possible to do this in Java?
>
> throw null;


Yes. JLS:

<http://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.18>


"throw Expression ;

The Expression in a throw statement must denote either 1) a variable or
value of a reference type which is assignable (§5.2) to the type
Throwable, or 2) the null reference..."

On 7/13/2012 10:29 AM, markspace wrote:
> On 7/13/2012 7:18 AM, bob smith wrote:
>> Is it possible to do this in Java?
>>
>> throw null;
>
>
> Yes. JLS:
>
> <http://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.18>
>
>
> "throw Expression ;
>
> The Expression in a throw statement must denote either 1) a variable or
> value of a reference type which is assignable (§5.2) to the type
> Throwable, or 2) the null reference..."

Interesting. Reading onward, though, we find

"If evaluation of the Expression completes normally,
producing a null value, then an instance V' of class
NullPointerException is created and thrown instead of
null. [...]"

.... which leaves a bit of a mystery. The O.P.'s example was
(fleshed out somewhat)

try {
throw null;
} catch (Exception e) {
// e is null here!
}

.... but according to the JLS that shouldn't have happened. Bug?

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

On 7/13/2012 7:48 AM, Eric Sosman wrote:

> Interesting. Reading onward, though, we find


Thanks for pointing that out. I'd guess there's no bug, just the OP
having a hard time interpreting his results.

On 7/13/12 8:22 AM, markspace wrote:
> On 7/13/2012 7:48 AM, Eric Sosman wrote:
>
>> Interesting. Reading onward, though, we find
>
>
> Thanks for pointing that out. I'd guess there's no bug, just the OP
> having a hard time interpreting his results.

Indeed.
class NullExceptionSSCCE {
public static void main(String[] str) {
try {
throw null;
} catch (Exception e) {
System.out.println("e = " + e);
}
}
}

=====
e = java.lang.NullPointerException
=====


If the OP was correct, it would have printed "e = null"

Without a real SSCCE from the OP, it's hard to say in what way the
results were misinterpreted.

On Fri, 13 Jul 2012 07:18:37 -0700 (PDT), bob smith
<bob@coolfone.comze.com> wrote, quoted or indirectly quoted someone
who said :

> } catch (Exception e) {
>
>and e is null?

Most likely you are misinterpreting the results. Try tracing the
program and put a breakpoint inside your catch.
--
Roedy Green Canadian Mind Products
http://mindprod.com
The greatest shortcoming of the human race is our inability to understand the exponential function.
~ Dr. Albert A. Bartlett (born: 1923-03-21 age: 89)
http://www.youtube.com/watch?v=F-QA2rkpBSY

markspace wrote:
> bob smith wrote:
> &gt; Is it possible to do this in Java?
> &gt;
> &gt; throw null;
>
>
> Yes. JLS:

Good place to get the final word, albeit with a bit of effort occasionally.

> <http://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.18>
>
>
> "throw Expression ;
>
> The Expression in a throw statement must denote either 1) a variable or
> value of a reference type which is assignable (§5.2) to the type
> Throwable, or 2) the null reference..."

Know. Don't guess. Read the JLS.

--
Lew

Eric Sosman wrote:
> markspace wrote:
> &gt; bob smith wrote:
> &gt;&gt; Is it possible to do this in Java?
> &gt;&gt;
> &gt;&gt; throw null;
> &gt;
> &gt;
> &gt; Yes. JLS:
> &gt;
> &gt; &lt;http://docs.oracle.com/javase/specs/...#jls-14.18&gt;
> &gt;
> &gt;
> &gt; &quot;throw Expression ;
> &gt;
> &gt; The Expression in a throw statement must denote either 1) a variableor
> &gt; value of a reference type which is assignable (§5.2) to the type
> &gt; Throwable, or 2) the null reference...&quot;
>
> Interesting. Reading onward, though, we find
>
> &quot;If evaluation of the Expression completes normally,
> producing a null value, then an instance V' of class
> NullPointerException is created and thrown instead of
> null. [...]&quot;
>
> ... which leaves a bit of a mystery. The O.P.'s example was
> (fleshed out somewhat)

No mystery. The first refers to a compiler rule, the second to a runtime rule.

> try {
> throw null;
> } catch (Exception e) {
> // e is null here!
> }
>
> ... but according to the JLS that shouldn't have happened. Bug?

It's no more a bug than that you might get a 'ClassCastException' when you cast
a value to another type. The compiler allows what the runtime sometimes
doesn't.

--
Lew

On Fri, 13 Jul 2012 13:11:18 -0700 (PDT), Lew wrote:

>> try {
>> throw null;
>> } catch (Exception e) {
>> // e is null here!
>> }

>> ... but according to the JLS that shouldn't have happened. Bug?
> It's no more a bug than that you might get a 'ClassCastException' when you cast
> a value to another type. The compiler allows what the runtime sometimes
> doesn't.

And there's really no way around that - Exception e = null; if
(externalCondition()) e = new Exception(); throw e; - can't very well
complain about that at compile time for arbitrary definitions of
externalCondition().

Liebe Gruesse,
Joerg

--
Ich lese meine Emails nicht, replies to Email bleiben also leider
ungelesen.

On 13/07/2012 4:55 PM, Joerg Meier wrote:
> On Fri, 13 Jul 2012 13:11:18 -0700 (PDT), Lew wrote:
>
>>> try {
>>> throw null;
>>> } catch (Exception e) {
>>> // e is null here!
>>> }
>
>>> ... but according to the JLS that shouldn't have happened. Bug?
>> It's no more a bug than that you might get a 'ClassCastException' when you cast
>> a value to another type. The compiler allows what the runtime sometimes
>> doesn't.
>
> And there's really no way around that - Exception e = null; if
> (externalCondition()) e = new Exception(); throw e; - can't very well
> complain about that at compile time for arbitrary definitions of
> externalCondition().

Well, there is one; "throw e;" *could* have been specified to compile
into what you presently get by compiling:

if (e == null)
throw new NullPointerException();
else
throw e;

--
public final class JSnarker
extends JComponent
A JSnarker is an NNTP-aware component that asynchronously provides
snarky output when the Ego.needsPuncturing() event is fired in cljp.

On 13/07/2012 21:09, Roedy Green allegedly wrote:
> On Fri, 13 Jul 2012 07:18:37 -0700 (PDT), bob smith
> <bob@coolfone.comze.com> wrote, quoted or indirectly quoted someone
> who said :
>
>> } catch (Exception e) {
>>
>> and e is null?
>
> Most likely you are misinterpreting the results. Try tracing the
> program and put a breakpoint inside your catch.

I'd rather say: if there's the slightest whiff of fishiness with what
your debugger tells you, throw it to the four winds and use logging instead.

In my observation debuggers are almost as often a source of confusion as
they are one of clarification. YMMV.

--
DF.

On Fri, 13 Jul 2012 23:06:36 +0200, Daniele Futtorovic
<da.futt.news@laposte-dot-net.invalid> wrote, quoted or indirectly
quoted someone who said :

>In my observation debuggers are almost as often a source of confusion as
>they are one of clarification. YMMV.

the main thing is to look from a slightly different angle.
--
Roedy Green Canadian Mind Products
http://mindprod.com
The greatest shortcoming of the human race is our inability to understand the exponential function.
~ Dr. Albert A. Bartlett (born: 1923-03-21 age: 89)
http://www.youtube.com/watch?v=F-QA2rkpBSY