为什么使用Collection< String> .class非法? [英] Why is using Collection<String>.class illegal?
问题描述
我对仿制药感到困惑。您可以声明如下字段:
I am puzzled by generics. You can declare a field like:
Class<Collection<String>> clazz = ...
您可以将此字段指定为:
It seems logical that you could assign this field with:
Class<Collection<String>> clazz = Collection<String>.class;
然而,这会产生错误:
令牌>上的语法错误,此令牌后预期无效
Syntax error on token ">", void expected after this token
所以它看起来像 .class
运算符不适用于泛型。所以我尝试了:
So it looks like the .class
operator does not work with generics. So I tried:
class A<S> { }
class B extends A<String> { }
Class<A<String>> c = B.class;
同样不起作用,生成:
类型不匹配:无法转换为
Class< Test.StringCollection> to Class< Collection< String>>
现在,我真的不明白为什么这不应该工作。我知道泛型类型没有具体化,但在这两种情况下,它似乎完全是类型安全的,无需访问运行时泛型类型。有人有想法吗?
Now, I really fail to see why this should not work. I know generic types are not reified, but in both cases it seems to be fully type safe without having access to runtime generic types. Anybody an idea?
推荐答案
泛型是不变的。
Object o = "someString"; // FINE!
Class<Object> klazz = String.class; // DOESN'T COMPILE!
// cannot convert from Class<String> to Class<Object>
根据您的需要,您可以使用通配符。
Depending on what it is that you need, you may be able to use wildcards.
Class<? extends Number> klazz = Integer.class; // FINE!
或许你需要这样的东西:
Or perhaps you need something like this:
Class<List<String>> klazz =
(Class<List<String>>) new ArrayList<String>().getClass();
// WARNING! Type safety: Unchecked cast from
// Class<capture#1-of ? extends ArrayList> to Class<List<String>>
至于运行时未确定的例如,你似乎掌握得很好,但无论如何,这是一个引用,来自关于泛型的Java教程, The Fine Print :通用类由其所有调用共享:
以下代码片段是什么打印?
What does the following code fragment print?
List <String> l1 = new ArrayList<String>();
List<Integer> l2 = new ArrayList<Integer>();
System.out.println(l1.getClass() == l2.getClass());
您可能想说 false
,但你错了。它打印 true
,因为泛型类的所有实例都具有相同的运行时类,而不管它们的实际类型参数如何。
You might be tempted to say false
, but you'd be wrong. It prints true
, because all instances of a generic class have the same run-time class, regardless of their actual type parameters.
也就是说,没有 List< String> .class
或列表<整数> .class
;只有 List.class
。
That is, there's no such thing as List<String>.class
or List<Integer>.class
; there's only List.class
.
这也反映在 JLS 15.8.2 Class Literals
类文字是一个表达式,由类,接口,数组或基本类型的名称或伪类型void组成,后跟
。
和令牌类
。
注意省略对泛型类型参数/参数的任何限制。此外,
Note the omission of any allowance for generic type parameters/arguments. Furthermore,
如果发生以下任何情况,则编译时错误:
It is a compile time error if any of the following occur:
- 命名类型是类型变量或参数化类型,或者是元素类型是类型变量或参数化类型的数组。
也就是说,这也不能编译:
That is, this also doesn't compile:
void <T> test() {
Class<?> klazz = T.class; // DOESN'T COMPILE!
// Illegal class literal for the type parameter T
}
基本上你不能将泛型与类文字一起使用,因为它没有意义:它们是未实现的。
Basically you can't use generics with class literals, because it just doesn't make sense: they're non-reified.
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