Re: Java language and library suggestions

From:

Tom Anderson <twic@urchin.earth.li>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 19 Jul 2009 23:24:32 +0100

Message-ID:

<alpine.DEB.1.10.0907192321240.13702@urchin.earth.li>

On Sun, 19 Jul 2009, Arne Vajh?j wrote:

Tomas Mikula wrote:

On Jul 19, 3:42 pm, Arne Vajh?j <a...@vajhoej.dk> wrote:

Tom Anderson wrote:

On Sun, 19 Jul 2009, Lew wrote:

Tomas Mikula wrote:

Anyway there are still many cases when one could use safely it to get
more readable code.

Arne Vajh?j wrote:

It can happen, but I don't think it occur frequently enough to
justify a feature that is so easy to misuse.

Tomas Mikula wrote:

I disagree again. Almost everything can be misused. If someone feels
like their code never throws an exception, they could tend to write an
empty exception handler:
try {
// code that is incorrectly assumed not to throw any exception
} catch(Exception e) { }
If the Exception can actually be thrown and should be handled, this is
very bad.
I guess that the following would be a much better (although still bad)
solution in this case.
@safe
// code that is incorrectly assumed not to throw any exception
So even if it's going to be misused, it could eventually restrain from
worse things.

"could" != "would".
The proposed language feature would be a change to the language that
would be easy to misuse, might just possibly (if you're right) help
ever-so-slightly in some corner cases, in order to save a little bit
of typing. It doesn't seem like a good tradeoff. Just write the damn
exception handler and quit complaining.

This *is* an exception handler! It's shorthand for:
try {
STATEMENT
}
catch (EXCEPTION e) {
throw new AssertionError(e);
}
How is that not an exception handler?

It is an exception handler.

But it is converting the exception that the designer of the API
being called consider a real possibility to an exception that should
never happen by the designer of the calling code.

The designer of the API may as well state that the declared exception
will only be thrown under certain circumstances. If I avoided these
circumstances, then the exception won't be thrown. I will provide an
example:

class WriterEncoder {
   public WriterEncoder(Writer w);

   /** @throws IOException if and only if the write() methods of
underlying Writer throw an exception. */
   public void writeEncoded(MyClass obj) throws IOException;
}

Now if I construct the WriterEncoder with StringWriter which does not
throw IOException on write, I can be sure that
WriterEncoder.writeEncoded() won't throw IOException either.

Yes.

But it is very bad code.

The safe construct is relying on knowledge about implementation of both
the calling and the called code instead of just relying on the exposed
API's.

But this is complete nonsense! That example DOES only rely on the exposed
APIs!

WriterEncoder.writeEncoded's API specifies that it throws the same checked
exceptions as the writer's write method. StringWriter.write declares that
it throws no checked exceptions. Therefore, if you use
WriterEncoder.writeEncoded with a StringWriter, you are permitted to
conclude that it won't throw any checked exceptions. That is not bad code,
and it is not relying on knowledge of implementation details.

tom

--
Hit to death in the future head