Generics method declaration confusion. Plz clarify

[Ahsan Habib]

Can any one please clarify me why this method signature below is wrong -

public static <A super Apple> void insertRipe(A apple, Basket<? extends A> basket)

It is a method which inserts only ripe apples into the basket. Apple sub-classes exist. The following signature is correct -

public static <A extends Apple> void insertRipe(A apple, Basket<? super A> basket)

Basket is a simple generic class as follows -

public class Basket<E> {
private E element;

public void setElement(E x) {
element = x;
}

public E getElement() {
return element;
}
}

Thanks in advance!

[Ankit Garg]

Originally posted by Ahsan Habib:
Can any one please clarify me why this method signature below is wrong -

public static <A super Apple> void insertRipe(A apple, Basket<? extends A> basket)

You can't use A super Apple, for super type bound you have to use ?..

public static <? super Apple> void insertRipe(A apple, Basket<? extends A> basket)

But then you can't use your code as you want as you will have to modify the complete method to look like this

public static <? super Apple> void insertRipe(? apple, Basket<? extends ?> basket)

but this will give you compilation errors...

[Ankit Garg]

Originally posted by M. Piva:

This is as generic method declaration, you cant' use ?.
The correct syntax is:

public <T> void method(T t)

If you would have read my post completely, then you would have realized that in the end I said that it will not compile....

[Marco Piva]

Yes, you are right.

[Ankit Garg]

@piva

why did you remove your posts??? Your second post was correct. It was the solution to the problem. post it back

also change your name. M. Piva is not a legal name here. So write your full name...

[Marco Piva]

Not being sure i prefered to delete it, anyway it was:

"The problem is that in the first code you're trying to modify a Basket object using the "? extends" wildcard.

When using either <? extends Class> or <?> wildcard, the collection can be accessed but NOT modified (compilation error).

When using a wildcard, List<? super Class>, the collection can be accessed AND modified."

[Aarav Thomas]

I'm not sure if the following example will make sense, but this is what i understood:
---------------------------------------------------
In case of <T extends String>, the compiler will replace all occurrences of T with 'String' (i.e. upper bound) with Type Erasure. And this makes perfect sense.

Had this been allowed (which is not) <T super String>, then the compiler has to replace all occurrences of T with Object; in which case even object of type Dog can be passed to T.