重写了“ Java有效”示例中通用方法的绑定不匹配

Question

class Main {
public static void main(String[] args) {
    List<Sub> list = new ArrayList<Sub>();

    Sub r = max(list);

    System.out.println(r);
}

static <T extends Comparable<? super T>> T max(List<? extends T> list) {
    return null;
}

private static class Super {
}

private static class Sub implements Comparable<Super> {
    public int compareTo(Super o) {
        return 0;
    }
}
}

Can anyone tell me why I'm getting compiler error at line "Sub r = max(list)"?

I'm reading Java Effective book and I got at, as they said, the most complex method declaration in that book. It's actually that max method.

Error is:

Bound mismatch: "The generic method max(List< ? extends T>) of type Main is not applicable for the arguments (List< Main.Sub>). The inferred type Main.Sub is not a valid substitute for the bounded parameter < T extends Comparable< ? super T>>"

Answer 1

您的Sub类必须扩展Super ：

private static class Sub extends Super implements Comparable<Super> {