Java泛型Wildcard

import java.util.HashSet;
import java.util.Set;

public class WildcardGeneric<E> {

    static int UnboudedGetSize(Set<?>s) {
        //s.add(1); 编译错误，只能add null
        s.add(null);
        return s.size();
    }

    int getSize(Set<E> s) {
        return s.size();
    }

    int BoundedGetSize(Set<? extends E> s) {
        return s.size();
    }
    /**
     * @param args
     */
    public static void main(String[] args) {
        Set<Integer> hs = new HashSet<Integer>();
        hs.add(10);

        System.out.println(UnboudedGetSize(hs));

        WildcardGeneric<Number> numberwg = new WildcardGeneric<Number>();
        //numberwg.getSize(hs); 编译错误，不支持Number的派生类
        System.out.println(numberwg.BoundedGetSize(hs));

        WildcardGeneric<Integer> integerwg = new WildcardGeneric<Integer>();
        System.out.println(integerwg.getSize(hs));
        System.out.println(integerwg.BoundedGetSize(hs));


    }

}