Submission: switch (...) instanceof feature

[Jeroen van Maanen]

I agree that "if (... instanceof ...)" is more readable than the extension of "switch" that I proposed originally. I believe that this feature that avoids an unnecessary new name and cast would be of great benefit to the Java language.

Jeroen

Derek Foster schreef:
> I notice that the example you gave is actually an argument for an "if" that respects instanceOf, rather than a 'switch' that does. It seems to me like an abuse of notation to use a 'switch' in a case like this where only one decision (it is or it isn't a subtype of a given type) is being made.
> 
> Although I am skeptical of the 'switch instanceof' feature, for the reasons I outlined below, I would very much like to see Java implement the 'if instanceof' feature that is implied in the proposal, and which various people have been suggesting, so that this very common coding pattern:
> 
> if (foo instanceof Bar) {
>      Bar bar = (Bar)foo;
>      doSomethingWith(bar);
> }
> 
> could be simplified to:
> 
> if (foo instanceof Bar) {
>      doSomethingWith(foo);
> }
> 
> with no loss of type safety. It seems to me that your example would be more readable with such a construct than it is with the 'switch instanceof' feature.
> 
> I had intended to submit a proposal along these lines myself, but simply ran out of time to put it together. Still, it seems to me that there's been widespread agreement on the list that the 'if instanceof' sub-proposal is a good idea.
> 
> Derek
> 
> 
> 
> 
> 
> -----Original Message-----
>> From: Ulf Zibis <Ulf.Zibis at gmx.de>
>> Sent: Apr 22, 2009 5:17 AM
>> To: Derek Foster <vapor1 at teleport.com>
>> Cc: Gabriel Belingueres <belingueres at gmail.com>, Jeroen van Maanen <jeroen at entreact.com>, coin-dev at openjdk.java.net
>> Subject: Re: Submission: switch (...) instanceof feature
>>
>> Am 22.04.2009 06:55, Derek Foster schrieb:
>>> Like Gabriel, I have severe reservations about this proposal. I am concerned that it will encourage people to avoid the common OOP "best practice" of using virtual method dispatch in favor of doing an explicit "switch" on each subtype of a type, in cases where doing so is not necessary.
>>>
>>> I concede that there are a few (FEW!) places where doing the equivalent of a switch on instances of a type is necessary (for instance, if the type is in a library you don't have the ability to change). I can see some value in this proposal for that purpose. However, I would very much not like to see use of this construct become commonplace. I already see too many instances of people doing the equivalent of "if (object.type == FOO) {doThis(); } else if (object.type == BAR) { doThat(); }" instead of writing "object.doSomething();"
>>>
>>>   
>> I like to provide an example where
>>
>> "object.doSomething();"
>>
>>  doesn't work:
>>
>>    synchronized Charset lookup(String lowCanonical) {
>>        // cache is initialized with available Charset classes names
>>        Object o = cache.get(lowCanonical);
>>        // Check cache first
>>        if (o instanceof String) {
>>            // Instantiate new charset
>>            Charset cs = newCharset((String)o, lowCanonical);
>>            // Cache it
>>            if (cs != null)
>>                cache.put(lowCanonical, cs);
>>            return cs;
>>        }
>>        return (Charset)o;
>>    }
>>
>>
>>  This would look much nicer, as it avoids casting:
>>
>>    synchronized Charset lookup(String lowCanonical) {
>>        // cache is initialized with available Charset classes names
>>        switch (Object o = cache.get(lowCanonical) instanceof ?) {
>>            case String :
>>                // Instantiate new charset
>>                Charset cs = newCharset(o, lowCanonical);
>>                // Cache it
>>                if (cs != null)
>>                    cache.put(lowCanonical, cs);
>>                return cs;
>>            case Charset :
>>                return o;
>>        }
>>    }
>>
>>
>> Refer:
>> https://java-nio-charset-enhanced.dev.java.net/source/browse/java-nio-charset-enhanced/tags/milestone4/src/sun/nio/cs/FastCharsetProvider.java?rev=684&view=markup
>> http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6790402
>> http://mail.openjdk.java.net/pipermail/coin-dev/2009-March/001182.html
>> http://mail.openjdk.java.net/pipermail/coin-dev/2009-April/001328.html
>>
>> - My example could also be smartly coded without switch..case statement, 
>> if we would enhance if statement accordingly
>> - I also would like to have possibility to declare variables in if 
>> clause, like it's possible in for clause:
>>
>>        if ((Object o = cache.get(lowCanonical)) instanceof String)
>>            ... ;
>>        else if (o instanceof Charset)
>>            ... ; 
>>
>>
>> -Ulf
>>
>>
>>> Derek
>>>
>>> -----Original Message-----
>>>   
>>>> From: Gabriel Belingueres <belingueres at gmail.com>
>>>> Sent: Mar 30, 2009 7:31 AM
>>>> To: Jeroen van Maanen <jeroen at entreact.com>
>>>> Cc: coin-dev at openjdk.java.net
>>>> Subject: Re: Submission: switch (...) instanceof feature
>>>>
>>>> IMO I'm against this.
>>>>
>>>> First, it is against current best practices for the design of
>>>> object-oriented software to make easier to code something with a case
>>>> statement on types/classes.
>>>>
>>>> Second:
>>>> void log(Object object) {
>>>>   switch (object) instanceof {
>>>>   case String:
>>>>     logger.debug("'" + object + "'");
>>>>   case Date:
>>>>     logger.debug(object.getTime());
>>>>   case void:
>>>>     logger.debug("null");
>>>>   default:
>>>>     logger.debug("<" + object.toString() + ">");
>>>>   }
>>>> }
>>>>
>>>> It think it is clearer (when possible) writing it with several
>>>> overloaded methods and double dispatching.
>>>>
>>>> ....
>>>>     
>