Question

My tests show that Java reflection cannot determine object types assigned at runtime to a generic object reference that uses a wildcard declaration. To overcome that problem, I wrote the following. How can this be improved? For starters, I get a raw type complaint when I declare a Class object but how else is this not a best practice example for determining the runtime type assigned to a wildcard reference?

class TestRuntimeType
{
    public static void main(String... args) throws Exception
    {
        Map<Integer, Inventory<?>> map = new HashMap<Integer, Inventory<?>>();
        Inventory<Shirt> janesShirts = new Inventory<Shirt>(new Shirt());
        janesShirts.add(new Shirt("jack", 17));
        map.put(0, janesShirts);

        CConsole.myPW.format("%s\n", map.get(0).getElementType());
    }
}

class Inventory<T> extends ArrayList<T>
{
    private Class elementType;

    Inventory(T example)
    {
        elementType = example.getClass();
    }

    Class getElementType()
    {
        return elementType;
    }
}

class Shirt
{   String maker; double size; 
    Shirt() {}
    Shirt(String maker, double size)
    { this.maker = maker; this.size = size; }
}

Answer 1

The best way to specify a class is to use that class. To avoid having to give the class twice you can use a factory.

class Inventory<T> extends ArrayList<T> {
    private final Class<T> elementType;

    private Inventory(Class<T> elementType) {
        elementType = elementType;
    }

    public Class<T> getElementType() {
        return elementType;
    }

    public static <T> Inventory<T> of(Class<T> elementType) {
        return new Inventory<T>(elementType);
    }
}

// later
Inventory<MyType> inv = Inventory.of(MyType.class);