Re: Separation of API and implementation

On 14.08.2007, Dmitry A. Kazakov <mailbox@xxxxxxxxxxxxxxxxx> wrote:
On Tue, 14 Aug 2007 14:32:28 -0000, hforco2@xxxxxxxxxxxxxx wrote:
But often hierarchies aren't not really parallel. Either X or Y could be
designed class-wide. Then you dispatch along the opposite axis and no
second dispatch (poorly emulated by a cast in your example) is needed.

Can you give an example of this - I don't fully understand what you
mean by 'class-wide'?

[..]
Consider X1, X2, X3 ... derived from X and Y1, Y2, ... derived from Y. foo
is defined for all possible combinations of Xi, Yj. Now there exist four
variants:

1. Full multiple dispatch is when for each combination there exists a
distinct implementation of foo. Java cannot this.
[..]

You cannot have this in Java due to numerous limitations of. So you have to
reduce it to either 2 or 3, or else emulate double dispatch in some quite
tricky way.


In Java this limitation can be partially walked round by using the
Visitor pattern for hierarchy Y while distpatching on X.
Of course all Y classes have to be under control of the writer of
the method - it won't work with 3rd party classes.

I've come across a project that claims to add multiple dispatch to Java
but I've never used it.
http://multijava.sourceforge.net/

--
Wiktor Moskwa
.

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