[403] in java-interest
Request for feature: type casing
daemon@ATHENA.MIT.EDU (Simon Spero)
Mon Jun 19 23:25:08 1995
Date: Mon, 19 Jun 1995 20:04:18 -0700 (PDT)
From: Simon Spero <ses@tipper.oit.unc.edu>
To: Java Interest Mailing List <java-interest@java.sun.com>
I've been working on some code that uses sub-classing to hold different
flavour requests that are parsed from a stream. There are some methods
that are specialed for each class, but in general sub-classing is being
used instead of old-fashioned unions (this is the 90s, dammit).
As is usually the case, there's a feature from lisp/clos that would make
this much easier; however unlike normal, the feature can be implemented
pretty easily without any overhead.
What I'd like to be able to do is something like:
class anObject {}
class anInt extends anObject {public int i;}
class aString extends anObject {public String s;}
....
anObject o = foo.parse(System.in);
typecase(o) {
case anInt:
total += o.i;
break;
case aString:
System.out.println(o.s);
break;
default:
throw(new RealityFailure_RebootUniverse());
}
This code can be defined in terms of existing control structures-
anObject tmp = o;
if(tmp instanceof anInt) {
anInt o = (anInt)tmp;
total += o.i;
} else if(tmp instanceof aString) {
aString o = (aString)tmp;
System.out.println(o.s);
} else {
throw new RealityFailure_Reboot_Universe();
}
The second form is a lot denser and harder to read; surely there's room
for just one bit of syntactic sweet and low in java yet (hmmm - though if
we had macros, we could add this ourselves :-)
Simon
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