Path: csiph.com!x330-a1.tempe.blueboxinc.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!nx02.iad01.newshosting.com!newshosting.com!69.16.185.11.MISMATCH!npeer01.iad.highwinds-media.com!news.highwinds-media.com!feed-me.highwinds-media.com!postnews.google.com!glegroupsg2000goo.googlegroups.com!not-for-mail
From: Lew <lewbloch@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: JNI generic type of jobject
Date: Tue, 4 Oct 2011 12:01:08 -0700 (PDT)
Organization: http://groups.google.com
Lines: 67
Message-ID: <10625927.131.1317754868953.JavaMail.geo-discussion-forums@prfc6>
References: <j6etsu$lkf$1@online.de> <4e8afbde$0$2529$e4fe514c@news2.news.xs4all.nl> <C5Fiq.2704$x14.849@newsfe22.iad>
Reply-To: comp.lang.java.programmer@googlegroups.com
NNTP-Posting-Host: 2620:0:1000:437c:224:d7ff:fe69:5838
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
X-Trace: posting.google.com 1317754869 25060 127.0.0.1 (4 Oct 2011 19:01:09 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: Tue, 4 Oct 2011 19:01:09 +0000 (UTC)
In-Reply-To: <C5Fiq.2704$x14.849@newsfe22.iad>
Complaints-To: groups-abuse@google.com
Injection-Info: glegroupsg2000goo.googlegroups.com; posting-host=2620:0:1000:437c:224:d7ff:fe69:5838; posting-account=CP-lKQoAAAAGtB5diOuGlDQk0jIwmH0T
User-Agent: G2/1.0
X-Google-Web-Client: true
Xref: x330-a1.tempe.blueboxinc.net comp.lang.java.programmer:8541

Daniel Pitts wrote:
>>  Philipp Kraus wrote:
>>> I use JNI calls for some Java classes. Some Java classes are generic
>>> classes like:
>>>
>>> class mytestclass<T> {
>>>
>>>   public native void mymethod();
>>>
>>> }
>>>
>>> The stub shows:
>>>
>>> JNIEXPORT void JNICALL Java_mytestclass_mymethod(JNIEnv* p_env, jobject
>>> p_object)
>>>
>>> How can I get from the jobject which object type is the generic
>>> parameter T? Because I would
>>> like to create different codes if I do something like:
>>>
>>>   mytestclass<int> x = new mytestclass<int>();
>>>   x.mymethod();
>>>
>>>   mytestclass<String> x = new mytestclass<String>();
>>>   x.mymethod();
>>
> int is not a valid generic type parameter, as int is a primitive and 
> generic types must be Object types.
> 
> Also, generics are not the same as C++ templates.  There isn't different 
> code created for each concrete usage. Its all exactly the same code.
> 
> If you are doing different behavior based on the compile time type, then 
> you need  to do a little bit more work to implement the strategy pattern.
> 
> class MyTestClass<T> {
>    private MyMethodStrategy<T> strategy;
> 
>    public mymethod() {
>       strategy.mymethod(this);
>    }
> }
> 
> interface MyMethodStrategy<T> {
>      void mymethod(MyTestClass<T> testClass);
> }
> 
> 
> class MyStringMethodStrategy implement MyMethodStrategy<String> {
>     public native void mymethod(MyTestClass<String> testClass);
> }
> 
> 
> class MyIntegerMethodStrategy implement MyMethodStrategy<Integer> {
>     public native void mymethod(MyTestClass<Integer> testClass);
> }
> 
> 
> Then you will have two different native methods to implement each strategy.

Kudos for a great answer!
+1

This pattern or ones like it are frequent and very helpful in generics programming.

-- 
Lew