Question

I have a class that maps incoming messages to matching readers based on the message’s class. All message types implement the interface message. A reader registers at the mapper class, stating which message types it will be able to handle. This information needs to be stored in the message reader in some way and my approach was to set a private final array from the constructor.

Now, it seems I have some misunderstanding about generics and / or arrays, that I can’t seem to figure out, see the code below. What is it?

public class HttpGetMessageReader implements IMessageReader {
    // gives a warning because the type parameter is missing
    // also, I actually want to be more restrictive than that
    // 
    // private final Class[] _rgAccepted;

    // works here, but see below
    private final Class<? extends IMessage>[] _rgAccepted;

    public HttpGetMessageReader()
    {
        // works here, but see above
        // this._rgAccepted = new Class[1];

        // gives the error "Can't create a generic array of Class<? extends IMessage>"
        this._rgAccepted = new Class<? extends IMessage>[1];

        this._rgAccepted[0] = HttpGetMessage.class;
    }
}

ETA:
As cletus correctly pointed out, the most basic googling shows that Java does not permit generic arrays. I definitely understand this for the examples given (like E[] arr = new E[8], where E is a type parameter of the surrounding class). But why is new Class[n] allowed? And what then is the “proper” (or at least, common) way to do this?

Answer 1

Java does not permit generic arrays. More information in the Java Generics FAQ.

To answer your question, just use a List (probably ArrayList) instead of an array.

Some more explanation can be found in Java theory and practice: Generics gotchas:

Generics are not covariant

While you might find it helpful to
think of collections as being an
abstraction of arrays, they have some
special properties that collections do
not. Arrays in the Java language are
covariant — which means that if
Integer extends Number (which it
does), then not only is an Integer
also a Number, but an Integer[] is
also a Number[], and you are free to
pass or assign an Integer[] where a
Number[] is called for. (More
formally, if Number is a supertype
of Integer, then Number[] is a
supertype of Integer[].) You might
think the same is true of generic
types as well — that List<Number>
is a supertype of List<Integer>, and
that you can pass a List<Integer>
where a List<Number> is expected.
Unfortunately, it doesn’t work that
way.

It turns out there’s a good reason it
doesn’t work that way: It would break
the type safety generics were supposed
to provide. Imagine you could assign a
List<Integer> to a List<Number>.
Then the following code would allow
you to put something that wasn’t an
Integer into a List<Integer>:

List<Integer> li = new ArrayList<Integer>();
List<Number> ln = li; // illegal
ln.add(new Float(3.1415));

Because ln is a List<Number>, adding
a Float to it seems perfectly legal.
But if ln were aliased with li, then
it would break the type-safety promise
implicit in the definition of li —
that it is a list of integers, which
is why generic types cannot be
covariant.