Java泛型之协变与逆变及extends与super选择

Number number = new Integer(1);  //correct

List<Number> list = new ArrayList<Integer>(1);  //error

List<? extends Number> list = new ArrayList<Integer>(1);  //corrent

Number[] numbers = new Integer[3]; 

List<? super Number> list = new ArrayList<Object>(1);  //corrent

public interface List<E> extends Collection<E> { boolean add(E e); }

List<? extends Number> list = new ArrayList<Integer>(); // correct
list.add(Integer.valueOf(1));  //error

List<? super Number> list = new ArrayList<Object>(); // correct
list.add(Integer.valueOf(1));  //correct

public static <T> void copy(List<? super T> dest, List<? extends T> src) {
    int srcSize = src.size();
    if (srcSize > dest.size())
        throw new IndexOutOfBoundsException("Source does not fit in dest");

    if (srcSize < COPY_THRESHOLD ||
        (src instanceof RandomAccess && dest instanceof RandomAccess)) {
        for (int i=0; i<srcSize; i++)
            dest.set(i, src.get(i));
    } else {
        ListIterator<? super T> di=dest.listIterator();
        ListIterator<? extends T> si=src.listIterator();
        for (int i=0; i<srcSize; i++) {
            di.next();
            di.set(si.next());
        }
    }
}

private static <E> E getFirst(List<? extends E> list){
    return list.get(0);
}

private static <E> void setFirst(List<? super E> list, E firstElement){
    list.add(firstElement);
}

public static void main(String[] args) {
    List<Integer> list = new ArrayList<Integer>();
    setFirst(list, 1);
    Number number = getFirst(list);
}