Cast entity with Java

I'm trying to upcast or downcast an entity in Java and then commit it to the database. For example, if cat is a specialization of animal:

    cat nowCat = (cat)animalParameter1;
    nowCat.settailLength(3);
    nowCat.setName("persian");
    nowCat.commit();

This doesn't work. I get

java.lang.ClassCastException: myfirstmodule.proxies.animal cannot be cast to myfirstmodule.proxies.cat at MyFirstModule.downcast (JavaAction : 'downcast_java')

Thanks for any ideas.