Java上下限&下界通配符 [英] Java Generics for Upper bound & lower bound wild cards

问题描述

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。

I was reading java generics, I came across an interesting query. My question is as follows.

1. 对于上限通配符

1. For an upper bounded wildcard

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

For a lower bounded wildcard

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文档我理解代码应该编译为点1以及

I am confused with this because looking at the Sun Oracle documentation I understand that the code should compile for point 1 as well

上限通配符
下限通配符<

Upper Bound Wildcard Lower Bound Wildcard

推荐答案

任何人都可以帮我理解吗？ >这是因为当你使用上限时，你不能向集合添加元素，只能读取它们。

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

这意味着这些是一些法律赋值：

this means that these are some of legal assignments:

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

，所以你不能保证当添加对象时，它将保存正确类型的对象。为了更好的解释，请按照：
如何我添加到List< ;? extends Number>数据结构？

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?

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