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From: Daniel Pitts <newsgroup.nospam@virtualinfinity.net>
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Subject: Re: JNI generic type of jobject
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Date: Tue, 04 Oct 2011 12:15:46 -0700
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On 10/4/11 12:01 PM, Lew wrote:
> 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.
>
They also provide greater flexibility for pluggable (user-defined) 
behavior.  When applicable, I often replace protected methods with 
strategy interfaces. Especially if any two protected methods are 
independent of one another in the same class.  Truth be told, I usually 
don't create protected methods at all any more, unless they are in some 
sort of adapter class for a "un-handled use-case" or some such.