java - 上限和下限通配符的 Java 泛型

Question

我正在阅读 java 泛型，我遇到了一个有趣的查询。我的问题如下。

对于上限通配符

public static void printList(List<? extends Number> list) {
    for (int i = 0; i < 10; i++) {
        list.add(i);// gives compilation error
    }
}

对于下界通配符

public static void printList(List<? super Integer> list) {
    for (int i = 0; i < 10; i++) {
        list.add(i);// successfully compiles
    }
}

我对此感到困惑，因为查看 Sun Oracle 文档我了解代码也应该针对第 1 点进行编译

谁能帮我理解这一点？

score 10 · Accepted Answer

This is because when you are using upper bound, you cannot add elements to collection, only read them.

this means that these are some of legal assignments:

List<? extends Number> l = new ArrayList<Integer>();
List<? extends Number> l = new ArrayList<Double>();

so you cannot guarantee that when adding object, it will hold correct types of objects. for better explatation please follow: How can I add to List<? extends Number> data structures?

score 2 · Accepted Answer

实际上，幸运的是，同样的情况，我在 Sun Oracle 文档的下一页中得到了答案。请找到下面的链接。可能对将来要搜索的人有用。

2 回答 2