Compilation Error when using Generics

[Ramu Ramaiah]

Hi Please find the code below.

package Mock;
 
public class Basket<E> {
private E element;
 
public void setElement(E x) {
element = x;
}
 
public E getElement() {
return element;
}
 
public String toString() {
System.out.println(element);
return "My Basket...";
}
}
 
class Apple {
}
 
class GoldenDelicious extends Apple {}
class Jonagold extends Apple {}
 
class AppleHelper {
public static boolean isRipe(Apple apple) {
return true;
}
 
public static boolean isRipeInBasket(Basket<? extends Apple> basket)
{
Apple apple = basket.getElement(); // 1
return isRipe(apple); // 2
}
 
}
 
class Test
{
public static void main(String[] args) {

Basket<? extends Apple> b1 = new Basket<GoldenDelicious>();
Basket<? extends Apple> b2 = new Basket<Jonagold>();
 
b1.setElement(new GoldenDelicious());
b2.setElement(new Jonagold());
 
AppleHelper help = new AppleHelper();
System.out.println(help.isRipeInBasket(b1));
System.out.println(help.isRipeInBasket(b2));
 
System.out.println("End...");
}
}

The below two calls to methods gives a compilation error.
b1.setElement(new GoldenDelicious());
b2.setElement(new Jonagold());

I don't understand why it is doing so, since the type of the baskets b1 and b2 is "anything which extends Apple".

[Ilja Preuss]

Notice that both b1 and b2 have the same type. So you can put exactly the same objects into both b1 and b2.

Do you really want that?

since the type of the baskets b1 and b2 is "anything which extends Apple"

No, that's not its type.

It's "a Basket with a type parameter which extends Apple", which is quite different. It could be a Basket<MyVerySpecialAppleWithWormsInIt> as far as the compiler is concerned.