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From: Philipp Kraus <philipp.kraus@flashpixx.de>
Newsgroups: comp.lang.java.programmer
Subject: Re: JNI generic type of jobject
Date: Tue, 4 Oct 2011 18:01:23 +0200
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On 2011-10-04 16:53:22 +0200, Daniel Pitts said:

> On 10/4/11 5:28 AM, Silvio wrote:
>> On 10/04/2011 02:23 PM, Philipp Kraus wrote:
>>> Hello,
>>> 
>>> 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();
>>> 
>>> Thanks
>>> 
>>> Phil
>>> 
>>> 
>> 
>> No such thing is possible. Generics are a compile time thing. At runtime
>> only the raw types are available.
>> 
>> Silvio
> 
> int is not a valid generic type parameter, as int is a primitive and 
> generic types must by 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.

This a very good solution, I implement my own pattern
Thanks