Java: Generics Inheritance

Question

I have the following interface for a marshaller that marshalls an Object to a DataNode object and back:

public interface DataMarshaller<T> {

    /**
     * returns the actual class that this marshaller is capable of marshalling.
     * @return 
     */
    Class<T> getDataClass();

    /**
     * marshalls the object to a DataNode
     * 
     * @param object
     * @return 
     */
    DataNode marshal(T object);

    /**
     * unmarshalls the object from a DataNode
     * 
     * @param node
     * @return 
     */
    T unMarshal(DataNode node);

}

To make sure that I'm able to get the correct marshaller for an object I also have the method Class<T> getDataClass() that returns its class (that would have been lost after compiling because of type erasure).

Now I wanted to implement this interface in a class that is able to marshal objects of type Octree<T> like this:

public class OctreeMarshaller<T> implements DataMarshaller<Octree<T>> {

    @Override
    public Class<Octree<T>> getDataClass() {
        return Octree.class; //<- compiletime error
    }

    @Override
    public DataNode marshal(Octree<T> object) {
        //...
    }

    @Override
    public Octree<T> unMarshal(DataNode node) {
        //...
    }


}

My problem is now that (of course) Octree.class is not of type Class<Octree<T>> but rather of the non generic type Class<Octree> and Java won't allow me to cast it to Class<Octree<T>>.

The question is now: is there a more elegent way to solve that than the two I already considered:

• change the return value in the interface from Class<T> to Class<?>
• change the class signature of OctreeMarshaller to public class OctreeMarshaller implements DataMarshaller<Octree>

Answer 1

Without an instance of Octree<T> you cannot construct a class object of type Class<Octree<T>>. But since we know (due to type erasure) there is only one class Octree during run time, it's safe to write

public class OctreeMarshaller<T> implements DataMarshaller<Octree<T>> {

    @Override
    @SuppressWarnings("unchecked")
    public Class<Octree<T>> getDataClass() {
        return (Class<Octree<T>>)(Class<?>)Octree.class;
    }

    [...]
}

Answer 2

With Class of a generic type, you need a double cast:

public Class<Octree<T>> getDataClass() {
    return (Class<Octree<T>>)(Class<?>)Octree.class;
}

Whacky, but it works.