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From: Robert Klemme <shortcutter@googlemail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: generic constructor call with concret type
Date: Wed, 05 Oct 2011 22:14:09 +0200
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On 10/05/2011 11:58 AM, Philipp Kraus wrote:
> On 2011-10-05 11:22:50 +0200, Robert Klemme said:
>
>> @SuppressWarnings("unchecked")
>> public Run(Class<? extends T> cl) {
>> if (cl == Float.class) {
>> helper = (Inner<T>) new DoFloat();
>> } else if (cl == Double.class) {
>> helper = (Inner<T>) new DoDouble();
>> } else {
>> throw new IllegalArgumentException("Invalid type: " + cl);
>> }
>> }
>>
>> @Override
>> public T doSomething() {
>> return helper.doAnother();
>> }
>>
>> // Test
>> public static void main(String[] args) {
>> final Base<Double> rd = new Run<Double>(Double.class);
>> final Base<Float> rf = new Run<Float>(Float.class);
>> System.out.println("Double: " + rd.doSomething());
>> System.out.println("Float: " + rf.doSomething());
>>
>> final Base<Integer> ri = new Run<Integer>(Integer.class);
>> System.out.println("Integer: " + ri.doSomething());
>>
>> // final Base<Integer> wontCompile = new Run<Integer>(Double.class);
>> }
>> }
>
> so I need a parameter for the type
>
>> Btw, what do you need that for?
>
> See my posting some days ago. I use a JNI call with C++ templates so I
> must set the template
> parameter on compile-time. Within the JNI call I can't determine the
> generic argument of the
> instantiated java object (because it is set on java-compile-time). So I
> create dual JNI calls one
> for float and one for double, but I must decide on the java ctor call if
> I instantiate the float or double
> object, you do this in the with if(cl == Float.Class), in my source code
> the DoFloat is the JNI call
> for the C++ template float (otherwise the double).

A simpler solution would be

public abstract class Base<T extends Number> {
   public static Base<Double> createDoubleHandler() { return new 
RunDouble(); }
   public static Base<Float> createFloatHandler() { ... }

   public T anylogic(T x) {
     //
     return x == null ? null : specific(x);
   }

   protected abstract T specific(T abc);
}

final class RunDouble extends Base<Double> {
   protected Double specific(Double x) {
     return 123d + x;
   }
}
...

Kind regards

	robert