generic constructor call with concret type

Hello,
I have th following proble:

abstract baseclass<T extends Number> {

    abstract T dosomething();
}

class runclass<T extends Number> implements baseclass<T>{}

Now I would only implementate the runclass<T> for the datatype Double
and Float on other Number-types the compiler
should create an error.

At next I use a inner interface and class like

class runclass<T extends Number> implements baseclass<T>{

    private interface innerintface<L extends Number>
    {
          public L doanother();
    }

    // and implement the interface with inner classes
    private class floatrun implements innerintface<Float>
    {
    }

    private class doublerun implements innerintface<Double>
    {
    }

   // use a private member to store the object
   private innerintface<T> obj = null

   public runclass() {
       // here is my problem to decide if I need create the obj = new
floatrun() or obj = new doublerun();
  }

}

My runclass<T> should be only work with Double or Float datatypes at
the moment. In C++ template
I would create the conrect implement types and a genereal type that
creates an assert. Can I do anything
like
class runclass<T only Double>?

Thanks

Phil