Re: How to create an instance of type T?

From:
Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 23 Feb 2008 08:33:39 -0800
Message-ID:
<47c04ae0$0$28586$7836cce5@newsrazor.net>
prowyh wrote:

I want to write a generic method that return the specified object
constructed with type parameter T and the parameter of type String.
Here is the code:

public class TTest
{
    public static void main(String[] args) throws Exception
    {
        GClass<Integer> i = new GClass<Integer>();
        int a = i.getValue("345").valueOf();

        GClass<Double> d = new GClass<Double>();
        double b = d.getValue("345.5").valueOf();
    }
}

class GClass<T>
{
    public T getValue(String p) throws Exception
    {
        // create and instance of type T or a Class class of type T
        // construct the object with p by reflection
        // return the object
    }
}

well, we can do the same thing by default(T) or
Activator.CreateInstance<T>() in C#. But how can I do that in Java?

Any hints appreciated. Thanks.


interface Factory<T> {
    T create();
}

Foo = new GClass<Foo>(new Factory<Foo>() { public T create() {return new
Foo(); } ).getValue("Bar");

You shouldn't use reflection unless you know very much what you are doing.

Also, note that Java doesn't have reified generic types, so you can't
get the Class object of your "T" parameter unless you have the caller
pass it in for you. If you can avoid it by using the idiom above
(called the factory pattern), it would be better. If you really wanted
to use reflection, then you could *also* create a ReflectionBasedFactory
<T> class that takes a Class<? extends T> object and uses reflection on
that to instantiate a class.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>

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