Factory Patterns with Generics

Question

I am trying to combine the good old factory pattern with generics and could use some advice.
I have the following
//Base class from which all my things extend
public abstract class Thing {
//one kind of thing
public class ThingOne extends Thing {
//another kind of thing
public class ThingTwo extends Thing {
//interface for my factory
public interface ThingFactory<T> {
public T create(long id);
//a factory for thingones
public class ThingOneFactory<T> implements ThingFactory<T> {
public T create(long id) {
return (T) new ThingOne();
public class TestClass{
public <T extends Thing> T getThing(ThingFactory<T> tf){
//do a bunch of generic stuff to figure out id to call create
ThingFactory<T> instance = tf;
return (T) instance.create(Long id);
}My calling code would know what kind of thing it needs but necessarily the things id. Which can be computed in a generic way.
TestClass gt = new TestClass();
gt.getThing(new ThingOneFactory<ThingOne>());This all seems to work properly but I am getting "Type safety: Unchecked cast from ThingOne to T" in my ThingOneFactory
My T's will always extend Thing, what am I missing? Is there a better way to do this?
Edited by: nedry on Dec 29, 2009 5:39 PM

Answer 1

I thought of that after I posted. But that just moves my unsafe cast warning into TestClass.Why?
return (T) instance.create(Long id);That can't have ever compiled. What is the exact code? And why did you have to cast (what was the warning/error)?