newInstance, generics, and "unchecked or unsafe operations"

From:
"Eric I." <rubytraining@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 14 Aug 2008 21:05:00 -0700 (PDT)
Message-ID:
<edc638b0-9d10-4b78-aaf2-92132dcbbe4e@z72g2000hsb.googlegroups.com>
Hi Everyone,

I'm trying to create a method in a class that takes some type of
genericized collection and creates a new, empty instance of it by
calling the newInstance method. I can get the code to work, but I do
get compiler warnings of "unchecked or unsafe operations".

Please note, I would like this code to be flexible, so whether the
method gets an ArrayList<Integer> or a LinkedList<String> it will work
and not generate compiler warnings.

In trying to hunt down a solution by reading comp.lang.java.programmer
and other online sources, I became aware of the issue of "type
erasure". In fact, at compile time I should know the full type of the
collection. Is there a way to "nicely" re-write this code, so it
doesn't generate the warning? If not "nicely" how about "nastily"?

====

import java.util.Collection;
import java.util.ArrayList;

class EmptyDuplicator<T>
{
    public Collection<T> duplicateEmptyVersionOf(Collection<T>
original)
           throws InstantiationException, IllegalAccessException
    {
        // COMPILER WARNING on next line; BUT I KNOW T at compile-
time!!!
        Collection<T> emptyVersionOf =
original.getClass().newInstance();
        return emptyVersionOf;
    }
}

public class Sample
{
    public static void main(String[] args)
           throws InstantiationException, IllegalAccessException
    {
        ArrayList<Integer> original = new ArrayList<Integer>();
        original.add(1);
        original.add(2);

        EmptyDuplicator<Integer> duplicator = new
EmptyDuplicator<Integer>();
        Collection<Integer> clone =
            duplicator.duplicateEmptyVersionOf(original);
        clone.add(4);

        System.out.println("original");
        for(int i : original) {
            System.out.println(i);
        }

        System.out.println("clone");
        for(int i : clone) {
            System.out.println(i);
        }
    }
}

====

Thanks in advance,

Eric

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