Java Generics Capture List<?>

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 co         


        

Answer 1

In your specific case, you can explicitly fix this:

public class WildcardError {
     void foo(List l) {
        // This will work
        l.set(0, l.get(0));
    }
}

Or if you don't want to change the original API, introduce a delegate helper method:

public class WildcardError {
    void foo(List l) {
        foo0(l);
    }

    private  void foo0(List l) {
        // This will work
        l.set(0, l.get(0));
    }
}

Unfortunately, the compiler cannot infer that "obvious" type. I've been wondering about that, too. It seems like something that could be improved in a compiler, as every wild card can be informally translated to an unknown type. Probably, there are some reasons why this was omitted, perhaps this is only intuition, but formally impossible.

UPDATE:

Note, I've just seen this peculiar implementation of Collections.swap():

public static void swap(List list, int i, int j) {
    final List l = list;
    l.set(i, l.set(j, l.get(i)));
}

The JDK guys resort to a raw type, in order to handle this, locally. This is a strong statement indicating that this probably should be supported by the compiler, but for some reason (e.g. lack of time to formally specify this) just wasn't done