Re: which foo(x)

From: Daniel Bonniot (Daniel.Bonniot_at_inria.fr)
Date: 12/17/03 

Date: Wed, 17 Dec 2003 20:31:53 +0100
To: Rick Trotter <rht325@yahoo.com>

Hi,

(Sorry for this late answer, I'm not reading the forum regularly)

> I was trying to write some polymorphic application code and found that
> a superclass method implementation gets invoked when I expect a
> subclass implementation. Here's how I have abstracted the problem.

You did a good analysis of the situation of Java. The key you seem to
miss to understand the "logics" behind Java's behaviour is the
following. In Java, a method call is of the form
   x.meth(a);
meth is a method name, and x is call the receiver of the call. There
could be more than one argument a, but it does not matter here.
The way the code to execute for this call is like this:
   1) during compilation, the compiler looks at the (static) type of a,
and looks in the class C which is the (static) type of x a method with
the name "meth" that matches the type of a (not relevant, but for the
sake of completeness: it also consider methods defined in super-classes
of C, and if there are more than one possible method, it will choose the
most specific). Let's call the result of this search method M.
   2) during execution, there is a mechanism to "refine" this search, by
looking at the actual class of the object that x represents. It is sure
that this class is either C, or a subclass of C. If it is a subclass D,
then a method with the same signature as M might have been defined in D,
or somewhere between C and D. In that case, that method will be called,
not M. However, if a method has a more precise signature than M because
it handles only a subclass of the original class for the argument, it is
not considered, even when this more precise method would be applicable
to the runtime argument of the call. It is this last situation that
surprised you in your example.

So you can see that there is a fundamental asymetry in a method call:
the type of the receiver will be checked during execution to find a more
precise method, but not the types of the arguments.

The behaviour of Java, C++, Eiffel and most object-oriented languages is
called "single dispatch" because they dispatch (choose the method to
execute) based on the type of a single argument. A few other OO
languages use "multiple dispatch", because, you guessed it, they use the
type of multiple arguments to decide what method to execute. To name a
few: Cecil, Dylan, CLOS. I am myself the main designer of a language
called Nice, which is a variation on Java that supports multiple
dispatch. For instance, thanks to multiple dispatch, you can write in Nice:

class Point
{
   private int x;

   equals(Point that) {
     return this.x == that.x;
   }
}

while Java programmers need to write:

class Point
{
   private int x;

   public boolean equals(Object that) {
     if (that instanceof Point) {
       Point thatPoint = (Point) that;
       return this.x == thatPoint.x;
     } else {
       return super.equals(that);
     }
   }
}

Both codes are stricly equivalent, but I believe the Nice version is
easier to write and to read ;-)

If you want to find out more, Nice's homepage is http://nice.sf.net

Daniel

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