WTF Java Syntax?

Can anyone explain a bit of Java Collections syntax to me?

1.  I created a class of things call JavaCard (a flash card) that extends Card.
1.  There is also a JavaCardDeck, subclassed from CardDeck.  
1.  CardDeck possesses an ArrayList called 'cards' as an iVar.  The ArrayList is declared as    possessing type <Card>.  I think that this is good polymorphism.
1.  One loads JavaCardDeck, which effectively wraps the ArrayList, with JavaCard

I want to be able to get a "normal" array out of a JavaCardDeck instance.  Thus:

aJavaCardDeck.getCards // should return an array of Cards, 
                       // makes sense given the way I defined the <>

Here's the code that's tricky:

 	public Object[] getCards(){
		return cards.toArray();
	}

From a runtime safety POV, this makes perfect sense.  Put everything into an Array of Objects.  
Good idea.  But *I* know, and each heap-being knows that it is effectively a JavaCard -- 
or a Card at the very least -- I would like to "cast" that array on the way out.

It appears that the way to do this is in ArrayLsit:

    public Object[] toArray(Object[] a) [Source](http://download.oracle.com/javase/1.4.2/docs/api/java/util/ArrayList.html#toArray(java.lang.Object[]))

Looks promising!

    Returns an array containing all of the elements in this list in the correct order; the runtime 
    type of the returned array is that of the specified array. If the list fits in the specified 
    array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the 
    specified array and the size of this list.

OK, great.  So I tried getting the syntax right a couple of times but eventually found this invocation ([Source](http://download-llnw.oracle.com/javase/tutorial/collections/interfaces/collection.html)):

    String[] a = c.toArray(new String[0]);

So here's my question.  How did one derive:
    String[] a = c.toArray(new String[0]);
from
    public Object[] toArray(Object[] a);
?

I could completely see replacing "Object" with "String...":

    public String[] toArray(String[] a)

But where does that zeroth-index come in?  I just don't get it and I want to understand what this syntax mean.  What's going on with *new String[0]*?

Thanks!