Re: generic constructor call with concret type

From:
Robert Klemme <shortcutter@googlemail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 5 Oct 2011 02:22:50 -0700 (PDT)
Message-ID:
<0dba7690-f51d-4b1f-bd59-92463784de71@20g2000yqq.googlegroups.com>
On Oct 5, 9:48 am, Philipp Kraus <philipp.kr...@flashpixx.de> wrote:

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>?


No, C++ templates != Java generics!

You could do this:

public class Run<T extends Number> implements Base<T> {

    private interface Inner<T extends Number> {
    T doAnother();
    }

    private static final class DoFloat implements Inner<Float> {
    @Override
    public Float doAnother() {
        return 123f;
    }
    }

    private static final class DoDouble implements Inner<Double> {
    @Override
    public Double doAnother() {
        return 456d;
    }
    }

    private final Inner<T> helper;

    @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);
    }
}

Btw, what do you need that for?

Kind regards

robert

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