泛型、数组和 ClassCastException [英] Generics, arrays, and the ClassCastException
问题描述
我认为这里一定有一些我不知道的微妙之处.考虑以下几点:
I think there must be something subtle going on here that I don't know about. Consider the following:
public class Foo<T> {
private T[] a = (T[]) new Object[5];
public Foo() {
// Add some elements to a
}
public T[] getA() {
return a;
}
}
假设您的主要方法包含以下内容:
Suppose that your main method contains the following:
Foo<Double> f = new Foo<Double>();
Double[] d = f.getA();
您将收到带有消息 java.lang.Object
cannot be cast to java.lang.Double
的 CastClassException
.
You will get a CastClassException
with the message java.lang.Object
cannot be cast to java.lang.Double
.
谁能告诉我为什么?我对 ClassCastException
的理解是,当您尝试将对象转换为无法转换的类型时会抛出该异常.也就是说,对于它不是实例的子类(引用文档).例如:
Can anyone tell me why? My understanding of ClassCastException
is that it is thrown when you try to cast an object to a type that cannot be casted. That is, to a subclass of which it is not an instance (to quote the documentation). e.g.:
Object o = new Double(3.);
Double d = (Double) o; // Working cast
String s = (String) o; // ClassCastException
看来我可以做到这一点.如果 a
只是一个 T
而不是数组 T[]
,我们可以得到 a
并在没有一个问题.为什么数组会破坏这个?
And it seems I can do this. If a
was just a T
instead of an array T[]
, we can get a
and cast it without a problem. Why do arrays break this?
谢谢.
推荐答案
Foo<Double> f = new Foo<Double>();
当你使用这个版本的泛型类 Foo 时,对于成员变量 a
,编译器本质上是采取这一行:
When you use this version of the generic class Foo, then for the member variable a
, the compiler is essentially taking this line:
private T[] a = (T[]) new Object[5];
并用 Double
替换 T
得到这个:
and replacing T
with Double
to get this:
private Double[] a = (Double[]) new Object[5];
您不能从 Object 强制转换为 Double,因此会出现 ClassCastException.
You cannot cast from Object to Double, hence the ClassCastException.
更新和澄清:实际上,在运行一些测试代码后,ClassCastException 比这更微妙.例如,这个 main 方法可以正常工作,没有任何异常:
Update and Clarification: Actually, after running some test code, the ClassCastException is more subtle than this. For example, this main method will work fine without any exception:
public static void main(String[] args) {
Foo<Double> f = new Foo<Double>();
System.out.println(f.getA());
}
当您尝试将 f.getA()
分配给 Double[]
类型的引用时会出现问题:
The problem occurs when you attempt to assign f.getA()
to a reference of type Double[]
:
public static void main(String[] args) {
Foo<Double> f = new Foo<Double>();
Double[] a2 = f.getA(); // throws ClassCastException
System.out.println(a2);
}
这是因为成员变量 a
的类型信息在运行时被删除了.泛型只在编译时提供类型安全(我在最初的帖子中以某种方式忽略了这一点).所以问题不是
This is because the type-information about the member variable a
is erased at runtime. Generics only provide type-safety at compile-time (I was somehow ignoring this in my initial post). So the problem is not
private T[] a = (T[]) new Object[5];
因为在运行时这段代码确实是
because at run-time this code is really
private Object[] a = new Object[5];
当方法 getA()
(在运行时实际返回一个 Object[]
)的结果被分配给 Double 类型的引用时,就会出现问题[]
- 此语句抛出 ClassCastException,因为 Object 不能转换为 Double.
The problem occurs when the result of method getA()
, which at runtime actually returns an Object[]
, is assigned to a reference of type Double[]
- this statement throws the ClassCastException because Object cannot be cast to Double.
更新 2:回答你的最后一个问题为什么数组会破坏这个?"答案是因为语言规范不支持泛型数组创建.查看此论坛帖子了解更多 - 为了向后兼容,什么都不知道关于运行时 T 的类型.
Update 2: to answer your final question "why do arrays break this?" The answer is because the language specification does not support generic array creation. See this forum post for more - in order to be backwards compatible, nothing is known about the type of T at runtime.
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