Re: Generics in Java 1.5 ( or is it java 5.0 ?... I always have confusion)

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 12 Jun 2008 12:44:24 -0700

Message-ID:

<48517c97$0$13373$7836cce5@newsrazor.net>

Vikram wrote:

Hi,
      Looking at the signature of the following method in
java.util.List

public interface List<E> extends Collection<E> {
.........
........
......

<T> T[] toArray(T[] a);
}

I wrote a small program as below:

    ArrayList<String> c = new ArrayList<String>();
    c.add("Vikram");
    c.add("Pyati");
    Integer[] i = new Integer[20];
    c.toArray(i);

This did not give me a compilation error, though it fails at runtime
giving java.lang.ArrayStoreException, which is perfect.

My question is , why did the above mentioned method be declared as
<E> E[] toArray(E[] a);
which will force the method to take only the array of formal type ( in
this case a String[] ) at the compile time

So, first off, Generics and Arrays don't always mix well.

Second, I think you're attempting to express this:
<T super E> toArray(T[] a);

Although, that breaks this:
List<Number> c = new ArrayList<Number>();
c.add(new Integer(3));
c.add(new Integer(15));

Integer[] i = c.toArray(new Integer[0]);

the toArray(Object[]) method is inherently a run-time only method,
because it can be used to create any type of array.

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