泛型类型的Java容器 [英] Java containers of generic types
问题描述
I am preparing for an exam and one of the codes we were given by professor is obscure to me:我准备参加考试，其中一位教授给我的代码很隐晦： Z {
static java.util.LinkedList<？扩展对象> a =
new java.util.LinkedList< String>（）;
public static void main（String [] args）{
a.add（null）; // 1
.add（new Object（））; // 2
a.add（new Object（））; // 3
System.out.println（a）; // 4
}
}
public class Z {
static java.util.LinkedList<? extends Object> a =
new java.util.LinkedList<String>();
public static void main(String[] args) {
a.add(null); // 1
a.add(new Object()); // 2
a.add("new Object()"); // 3
System.out.println(a); // 4
}
}
NetBeans在这里给我提供了非常奇怪的编译错误：
NetBeans gives me really strange compilation errors here:
no suitable method found for add(java.lang.Object)
method java.util.LinkedList.add(capture#1 of ? extends java.lang.Object) is not applicable
(actual argument java.lang.Object cannot be converted to capture#1 of ? extends java.lang.Object by method invocation conversion)
no suitable method found for add(java.lang.String)
method java.util.LinkedList.add(capture#2 of ? extends java.lang.Object) is not applicable
(actual argument java.lang.String cannot be converted to capture#2 of ? extends java.lang.Object by method invocation conversion)
任何人都可以向我解释这些错误吗？我非常确定应该可以将String对象添加到此列表中。
Could anyone please explain me these errors? I was pretty sure that it should be possible to add String objects to this list.
推荐答案
PECS - Producer扩展，Consumer super - 这是布洛克提出的一个助记符，以应付上述情况。
PECS - Producer extends, Consumer super - this is a mnemonic proposed by Bloch to cope with the above.
换句话说，如果您使用 extends 您只能从您的收藏中产生元素。如果你想要它消耗元素 - 使用 super 。
In other words, if you use extends you can only produce elements from your collection. If you want it to consume elements - use super.
关键是强制编译时安全。如果您的收藏是使用定义的？扩展SomeBaseClass ，这意味着它可以包含基类的单个子类的实例。在你的情况下，你正在定义新的LinkedList< String>（），但你尝试添加一个 Object - 这应该不被允许，并且被编译器捕获。
The point of that is to enforce compile-time safety. If your collection was defined with ? extends SomeBaseClass, this would mean "It can contain instances of a single subclass of the base class". In your case you are defining new LinkedList<String>(), but you try to add an Object - this should not be allowed, and it is caught by the compiler.
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