Question

I was looking at the Java Generics documentation and found this piece of code,

public class WildcardError {

void foo(List<?> l) {
    //This give a compile time error
    l.set(0,l.get(0));
}
}

I can understand that we are fetching an element from a List<?> and trying to set it to another List<?>. So the compiler gives an error. My question is it makes sense when the 2 lists are different i.e. l.set(0, m.get(0)) here lists l and m are different. But in the above example, l and l are the same lists. Why isn’t the compiler smart enough to see that? Is it hard to implement it?

Edit:
I am aware that I can fix it by a helper method or by using T instead of a ?. Just wondering why compiler doesn’t do it for me.

Answer 1

The compiler reports an error because there is no way — in general — that it can tell whether two expressions, (in this case l and l) refer to the same list.

Related, somewhat generalized, question:

• How does the JLS specify that wildcards cannot be formally used within methods?