Re: how to instantiate new return object of generic type

From:

Robert Klemme <shortcutter@googlemail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 16 Apr 2007 15:23:32 +0200

Message-ID:

<58hbm5F2gdt9fU1@mid.individual.net>

On 16.04.2007 14:49, tom forsmo wrote:

Hi

I have a problem with generics I cant seem to solve.

Consider this example pseudo-code:

public class Table<K, V> {

    public class Elem<V> {

        String key;
        V value;

        public getValue();
    }

    public V getElem(key, locale) {

        Elem e = findElem(key);

        V val = e.getValue(key);

        return ( val )
    }
}

I want to add to some code to getElem() (not findElem()) that creates a
completely new val instance/value of type V under certain circumstances.
The problem is, how do I do this when I dont know the actual type of V is?

You can either use reflection or you define a generic interface for a
factory object that will create a new object from the given instance.
Something along the lines of

interface Factory<V> {
V create(V template);
}

Then your Table needs a member with this type to which the creation is
delegated. A generic default implementation would probably just return
the argument.

I'd use the factory approach as it is more flexible and simpler.

(For bizarre reasons this is the only way to do it, I think. The
underlying functionality of findElem() can not be extended/changed to do
what I want, because that would require an entire project of its own.
But if anybody has any other suggestion on how to solve the problem
without starting a new subproject, I'd be happy to hear it.)

Kind regards

robert