Re: How to check if object is an instanceof a class, but not a
sub-class ?
On Fri, 25 Jun 2010, Simon Brooke wrote:
On Thu, 24 Jun 2010 12:45:02 +0000, Andreas Leitgeb wrote:
Eric Sosman <esosman@ieee-dot-org.invalid> wrote:
On 6/23/2010 5:27 PM, Daniel Pitts wrote:
[... using getClass() in equals() ...] Actually, I would use:
if (other == this) { return true; }
if (other == null || other.getClass() != getClass() { return false; }
Hmm. I usually write
if (other == null || other.getClass() != Thing.class)
Is there a reason to prefer one or the other?
I'm rather surprised to see .equals() being called an exceptional
usecase for exact class-check.
From previous discussions here and elsewhere, I had gathered, that
the "really correct" way to handle (non-trivial!) .equals() in a class
hierarchy would be to make all non-leaf-classes abstract.
Making all non-leaf classes abstract is an example of poor design.
On the contrary, it's a sign of well-factored code. It's not an iron rule,
but in general, if you have concrete classes extending other concrete
classes, there are abstractions you haven't identified.
public class Bird {
public boolean getCanFly() {
return true;
}
}
public class Penguin extends Bird {
public boolean getCanFly() {
return false;
}
}
Are you saying that class Bird ought not to be instantiable? Are you
saying I ought to add a class:
public class InstantiableBird extends Bird {} ?
which has no new semantic content, which overrides or specialises
nothing, and that's 'really correct'?
In this example, it's hard to say, because modelling kinds of birds as an
inheritance hierarchy is itself not good OO design. The idea of mapping
zoological hierarchies onto executable ones is - if you'll excuse the
expression - a canard. This would be a time to use the Type Object
pattern, and push the taxonomy out into a set of flyweight instances of a
BirdType class.
But sticking with the example, yes, you should pull out more classes:
Bird
FlyingBird
Swallow
AfricanSwallow
EuropeanSwallow
FlightlessBird
Penguin
With the obvious abstract and concrete methods.
tom
--
Thinking about it, history begins now -- sarah