了解Java泛型中的通配符 [英] Understanding wildcards in Java generics

问题描述

我不确定为什么以下代码中的最后一条语句是非法的。 Integer 应该是？的子类型，为什么我不能将它分配给 b ？

I'm not sure why the last statement in the following code is illegal. Integer should be a subtype of ?, so why can't I assign it to b?

List<String> a = new ArrayList<String>();
a.add("foo");

// b is a List of anything
List<?> b = a;

// retrieve the first element
Object c = b.get(0);
// This is legal, because we can guarantee
// that the return type "?" is a subtype of Object

// Add an Integer to b.
b.add(new Integer (1)); 

推荐答案

关键是 b 指的是 类型的列表，但编译器不知道类型是什么，所以它不知道是否是向它添加 Integer 是有效的。还有一件好事，就是你的例子 - 你将为最初创建的对象添加一个 Integer 来保存一个字符串列表。当然，Java在执行时会丢失这些信息，但编译器会尽可能保持安全。

The point is that b refers to a list of some type, but the compiler doesn't know what the type is, so it doesn't know whether or not it's valid to add an Integer to it. And a good thing too, given your example - you'd be adding an Integer to an object initially created to hold a list of strings. Sure, that information is lost at execution time in Java - but the compiler tries to keep you as safe as it can.

请参阅有关 lot 的Java泛型常见问题解答 更多信息。

See the Java generics FAQ for a lot more information.

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