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From: Eric Sosman <esosman@comcast-dot-net.invalid> (Eric Sosman)
Newsgroups: comp.lang.java.programmer
Subject: Re: Two exceptions
Date: Sat, 09 Dec 2017 14:55:38 +1200
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On 12/9/2017 11:15 AM, Stefan Ram wrote:
> Victor Porton <porton@narod.ru> writes:
>> So I have two alternatives:
>> 1. Derive ParseException from FatalParseException what seems to do the right
>> thing but seems illogical.
>
>    The type of a result of a parse is
>
> class ParseResult { ...
>
>    which can have subclasses
>
> class ProperParseResult extends ParseResult { ...
> class ErrorParseResult extends ParseResult { ...
>
>    which in turn can have several subclasses each of which
>    might implement several interfaces.
>
>    A parseResult might contain other parseResults (AST).

     This would work, yes, but I think it would lead to clumsy
code at the call site, something like

        ParseResult result = parse();
        if (result instanceof ProperParseResult) {
           ProperParseResult ppr = (ProperParseResult) result;
           ...
        } else if (result instanceof ErrorParseResult) {
            ErrorParseResult epr = (ErrorParseResult) result;
            ...
        } else {
            throw new IllegalStateException("bad result", result);
        }

That is, you'd need to use `instanceof' (or equivalent) to learn what kind of a
 result you obtained -- and if you find yourself using `instanceof' a lot,
that's usually a sign that the design could be improved.

     (It can be argued that `try/catch' is a disguised `instanceof',
particularly if there are multiple `catch' clauses.  This seems forgivable,
though, on the grounds that exceptions are "unusual.")

--
esosman@comcast-dot-net.invalid
Eleven hundred thirty-eight days to go.