java - Casting an array of Objects into an array of my intended class

Question

Just for review, can someone quickly explain what prevents this from working (on compile):

private HashSet data;

...

public DataObject[] getDataObjects( )
{
    return (DataObject[]) data.toArray();
}

...and what makes this the way that DOES work:

public DataObject[] getDataObjects( )
{
    return (DataObject[]) data.toArray( new DataObject[ Data.size() ] );
}

I'm not clear on the mechanism at work with casting (or whatever it is) that makes this so.

Answer 1

Because toArray() creates an array of Object, and you can't make Object[] into DataObject[] just by casting it. toArray(DataObject[]) creates an array of DataObject.

And yes, it is a shortcoming of the Collections class and the way Generics were shoehorned into Java. You'd expect that Collection<E>.toArray() could return an array of E, but it doesn't.

Interesting thing about the toArray(DataObject[]) call: you don't have to make the "a" array big enough, so you can call it with toArray(new DataObject[0]) if you like.

Calling it like toArray(new DateObject[0]) is actually better if you use .length later to get the array length. if the initial length was large and the same array object you passed was returned then you may face NullPointerExceptions later

I asked a question earlier about Java generics, and was pointed to this FAQ that was very helpful: http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html