generics and arrays and multi-class collections

 xen <>
Sat, 29 Sep 2007 17:38:06 -0700
Yo peoples of the earth,

What is the proper use of generics when you want to use arrays as
Is there anybody out there that writes code that does NOT generate
unchecked warnings?

For example, java.util.ArrayList uses an array of Object to store its
elements, and uses a cast (E)elements[i] to retrieve them, which
generates an unchecked warning. I myself have wanted to use an array
of generified sets, so I just create a Set[] and assign it to a
Set<E>[], which generates a warning. I can trim down on the warnings
so that I only get "unchecked conversion" and "unchecked cast"
warnings, but it seems I'll have to live with those.

My last addition was a Map that can store Lists that store different
classes that are all decendants of a superclass:

    Map<Class<? extends Feature>, List<? extends Feature>> stores;
    stores = new HashMap<Class<? extends Feature>, List<? extends

I want a method that retrieves each store, and creates one if not

    public <F extends Feature> List<F> getStore(Class<F> c) {
        List<F> L = (List<F>)stores.get(c); // unchecked cast
        if (L == null) {
            L = new ArrayList<F>();
            stores.put(c, L);
        return L;

The cast generates an unchecked warning.

Now given
    class Block extends Feature {}
I can do
    List<Block> = getStore(Block.class);
which is what I wanted.

What are your experiences, any advice?

greetings, xen.

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