Subtype in Generics?

[dmjpro]

List<Number> li1 = ..;
List<Integer> li2 = ..;
li1 = li2; //[B]li1[/B] can't be subtype of [B]li2[/B]

Explain: If li1 gets updated with the Sub Class of Number and which is not integer then accessing li2 results runtime error.

Then ..

List<?> li1 = ..;
li1 = li2; //it's valid

Here you can't update li1 except null.

List<? extends Number> li1 = ..;
li1=li2; //it's also valid

Here you can't also update li1 except null for the same reason(explaine d earlier).

So with the valid codes li1 can't be updated. Then why ? extends SuperClass. What's the problem with first code fragment?

[JosAH]

Read (especially) Chapter 3 from this link.

kind regards,

Jos