Re: About using assertion
On 05/19/2011 10:41 AM, Robert Klemme wrote:
On 19 Mai, 15:38, Lew<no...@lewscanon.com> wrote:
On 05/19/2011 09:16 AM, Michal Kleczek wrote:
Lew wrote:
That is wrong. That needs to be checked with an 'if' if 'NaN' is not a
legitimate input. 'assert' is not meant to correct things, only to make
sure
I totally agree with you.
that the correction is documented and proven. If 'assert' is turned off
in production, you still have the bug if you use 'assert' as the only
guard.
I was imprecise - just wanted to praise an example of "having confidence"
being sooo wrong and how having assertions (even though they look redundant)
_may_ help you.
Totally correct.
Assertions are a signpost: HERE THERE BE AN INVARIANT!
When subtypes or refactored code violate the invariant, the assertion can help
catch it.
public class Foo
{
private Attribute attribute;
public void setAttribute( Attribute attr )
{
if ( attr == null )
{
throw new IllegalArgumentException( "null argument, dummy" );
}
attribute = attr;
assert attribute != null;
}
public Attribute getAttribute()
{
assert attribute != null;
return attribute;
}
}
public class Bar extends Foo()
{
public void setAttribute( Attribute attr )
{
attribute = attr;
Sorry for the nitpicking, but this cannot compile unless you make
"attribute" protected or package visible. But that in itself would be
a code smell IMHO. (Even though Java's std lib uses protected in
various places. But that's a completely different topic...)
Good call.
}
}
The assertion will help reveal that class 'Bar' violated the invariant. In
production, the 'NullPointerException' (in the logs from 'getAttribute()'
clients) will trigger an investigation, which will include enabling assertions
for the class that yields the value, and then the invariant checks will help
find the problem.
I think I get your point about the usage of assert here but we might
find a better example. In this particular case the NPE would catch
misbehaved sub class code anyway which must go through setAttribute if
the member variable remains declared "private" (my preferred
solution).
How about this as an example using template method pattern.
public abstract class Base {
/** Sub classes must override this and return an Attribute
* where {@link Attribute#getProperty()} returns something
* useful (!= null).
* @return an Attribute with Property, not null.
*/
protected abstract Attribute getAttribute();
/** This is the great algorithm this class is all about. */
public void theAlgorithm() {
final Attribute at = getAttribute();
/* We use individual asserts instead of&& to easier find the
* violated rule. */
assert at != null;
assert at.getProperty() != null;
// now we can start working
System.out.println("Now this works: " +
at.getProperty().getLength());
}
}
My point would've been better made if I'd referred to refactoring instead of
subclassing. Your example is better. I do worry about the use of 'assert' to
check the invariant without backing logic, OTOH this is a use case where the
superclass seeks to avoid the overhead of runtime checking when the
implementor is supposedly following the documentation. So it's a really good
example.
Because the method is protected, it's part of the API contract for 'Base', so
a paranoid author would have put 'if' checks, possibly exceptions into
'theAlgorithm()'. But it's only part of the contract for programmers who live
close to the implementation, so it's perfectly reasonable to insist that the
subclass handle all the 'if' checks, and that the superclass have just
'assert' to provide a safety net. An NPE at the 'println()' call would hint
to ops to enable assertions, then the problem diagnosis is simple via those
assertions.
This will now be part of my pedagogical toolkit for explaining 'assert'.
Thank you.
--
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg