How do I correctly use generics in this case?

Kid101 :

Below is my Transformer Interface

public interface Transformer<BusinessObject, O extends State>
{
    public O transformToState(BusinessObject input);
}

This is one of my Transformer Impl

public class GoldTransformer implements Transformer<BusinessObject, Gold>
{
    @Override
    public Gold transformToState(BusinessObject input) {
        GoldBO goldbo= (GoldBO) input; // redundant casting line
        //do some transformation with BO to make it a state with some business logic
    }
}

Here is my another Transformer Impl

public class SilverTransformer implements Transformer<BusinessObject, Sliver> 
{
    @Override
    public Gold transformToState(BusinessObject input) {
        SilverBO goldbo= (SilverBO) input; // redundant casting line
        // again do some transformation with BO to make it a state with some business logic
    }
}

Both SilverBO and GoldBO Implements BusinessObject which is a marker interface. And Silver and Gold extend State. I really find the casting annoying and redundant is there a better way to use generics here? or a better pattern to use? I don't want to make any changes to state i.e. gold and silver.

Mureinik :

You could generalize the interface on the input BusinessObject too:

public interface Transformer<I extends BusinessObject, O extends State> {
    public O transformToState(I input);
}

public class GoldTransformer implements Transformer<GoldBO, Gold> {    
    @Override
    public Gold transformToState(GoldBO input) {
        // Code...
    }
}