Object cast in Prototype pattern

[vitesse wei]

I just go through the prototype pattern with a shallow clone implementation,code as bellow:

class Product implements Cloneable{
	int[] data = {1,2,3,4};
	public Object Clone(){
		try {
			return super.clone();
		} catch (CloneNotSupportedException e) {
			return null;
		}
	}
}
public class Test {
	public static void main(String[] args){
		Product p1 = new Product();
		System.out.println(p1.data.toString());
		Product p2 = (Product)p1.Clone();
		System.out.println(p2.data.toString());
	}

}

in Clone method,we call super.clone(),that return a Object,this Object is not a Product and we know that Product is a Object,how can we cast the Object to Product in main body without error?,
thanks ranchers

[Rob Spoor]

vitesse wei wrote:in Clone method,we call super.clone(),that return a Object,this Object is not a Product and we know that Product is a Object,how can we cast the Object to Product in main body without error?,
thanks ranchers

Object.clone() returns an object of the current type. Since Product overrides Object, super.clone() does return a Product instance, it's only the reference that's Object. Your code should compile just fine. Alternatively, using a covariant return type (available since Java 5.0), you move the cast to the method:

class Product implements Cloneable{
	int[] data = {1,2,3,4};
	public Product Clone(){ // covariant return type
		try {
			return (Product)super.clone(); // cast here
		} catch (CloneNotSupportedException e) {
			return null;
		}
	}
}
public class Test {
	public static void main(String[] args){
		Product p1 = new Product();
		System.out.println(p1.data.toString());
		Product p2 = p1.Clone(); // no more cast here
		System.out.println(p2.data.toString());
	}

}

Just two notes:
1) The clone will share the same array instance. The reference is copied, not the array itself. Modifying the content of p1.data will modify p2.data as well.
2) Calling toString() on an array produces a String that's usually not very useful. java.util.Arrays.toString(p1.data) will return a more readable String.

[vitesse wei]

Thanks, Saloon.