[516] in java-interest
Re: Multiple return values
daemon@ATHENA.MIT.EDU (Peter Kessler)
Tue Jun 27 13:53:12 1995
Date: Tue, 27 Jun 1995 10:08:42 -0700
From: Peter.Kessler@Eng.Sun.COM (Peter Kessler)
To: java-interest@java.Eng.Sun.COM
In the IDL-to-Java compiler I've written, I handle `out' and `inout'
parameters (which are equivalent to multiple return values) by generating,
for each user-defined IDL type, a Holder class with a public instance
variable of the user-defined type (and for convenience, a constructor that
takes an instance of the user-defined type).
Then `out' and `inout' parameters are mapped as instances of the Holder
classes. Before the call the caller has to initialize the Holder instance
for an `inout' parameter, and the callee can change the value of the
instance variable inside the Holder before returning for an `out' or
`inout' parameter. In the IDL support classes I've also defined Holders
for all the IDL basic and built-in types.
I think there was a `style' in some Java code to pass arrays of length
1 in places where in other languages you would use pointer or reference
parameters. I thought that was confusing, and since I'm generating the
Java code from IDL declarations, it's not much extra work to also generate
the Holder classes.
I tried a mapping in which I defined, in the style of the Mesa
programming language, a `return record' class for each IDL operation that
returned more than one result. That seemed to work just as well as Holder
classes. The difference is in the perturbation in the client code when you
change an operation from returning a single result (as the result of the
mapped Java method) to returning multiple results (e.g. with `out'
parameters, via a return record result). With Holder classes you just add
the new parameter as a Holder instance. With return records you have to
change the way you access the original result of the operation, which I
thought was an unnecessary complication.
... peter
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