Question

Suppose that we have:

abstract class Item {   public Item(){}}
abstract class ExtraItem extends Item {  public ExtraItem(){}}
class A extends Item{ public A(){}}
class B extends Item{ public B(){}}
class C extends Item{ public C(){}}
class EA extends ExtraItem{ public A(){}}
class EB extends ExtraItem{ public B(){}}
class EC extends ExtraItem{ public C(){}}

I want to write factory class like this but without casting:

   class Factory {

       Item createItem(String className){
                Class clazz = Class.forName(className);
                return (Item) clazz.newInstance(); //I don't want here cast to Item    
    }
    ExtraItem createExtraItem(String className){
               Class<? extends ExtraItem> clazz = (Class<? extends ExtraItem>)   Class.forName(className); // I don't want here cast to Extraitem
                return clazz.newInstance();
        }
    }

I try write it as above (of course with cast):

@SuppressWarnings("unchecked")
public static <T extends Item> T createItem(String protocolType) {
    try {
        String className = protocolType;
        Class<?> clazz = Class.forName(className);                        
        return (T) clazz.newInstance();
    } catch (Exception e) {
        Log.e("ItemFactory", "Lack key \"" + protocolType + "\" in map: " + e.getLocalizedMessage());
    }
    return null;
}

but how use here Class.cast() method? It a better way?

Answer 1

If it’s simply that you don’t like the compiler warnings then you can use Class.asSubclass to do the check for you:

Class<? extends Item> clazz = Class.forName(className).asSubclass(Item.class);

And the same principle for ExtraItem. This will throw a ClassCastException at runtime if the class named is not actually a subclass of Item.