Why the following doesn't compile

Question

Can someone explain me why the following doesn't compile:
class A<N extends Number> {
    public LinkedList<String> list;
    public N getN() {
        return null;
class Test {
    Test(A a) {
        Number n = a.getN();
        String str = a.list.getFirst();
}The compiler error is:
incompatible types
found   : java.lang.Object
required: java.lang.String
        String str = a.list.getFirst();And if compiler "lose" generic info for some reason or other, so it don't know type "a.list.getFirst()" anymore and think that it's Object, then why it know type of "a.getN()"?

Answer 1

DrClap wrote:
I'm not convinced by that explanation. I've used List<Whatever> in classes which aren't themselves generic and it works as I and the OP expect it to work. And I don't really see the difference between a class which isn't generic and a class which could be generic but isn't. I also don't see why public members should be treated differently when accessed from outside a class compared to when accessed inside a class.
If it indeed doesn't compile (and I haven't tested that myself) then no doubt there's a reason for that, this isn't one of those convoluted examples which the compiler writers could have missed without anybody noticing. It just doesn't seem to me that what jtahlborn wrote is actually the reason.Jtahlborn was correct with his explanation: whenever you use a generic type as the corresponding "raw type", the type loses all its generic features.
Thus, the public member "list" loses its parameter of type String and becomes the raw type "List"
The method "getN" loses its return type +"N extends Number"+ and yields simply a "Number".
For a detailed discussion, see [http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#110257]