Java 不兼容的类型和新的类型变量
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原文地址: http://stackoverflow.com/questions/20543966/
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incompatible types and fresh type-variable
提问by Marcus Junius Brutus
I get the following compilation message:
我收到以下编译消息：
[javac] ... error: incompatible types
[javac] exceptionClassHolder = new Holder<>( (new Exception()).getClass() );
[javac] ^
[javac] required: Holder<Class<? extends Exception>>
[javac] found: Holder<Class<CAP#1>>
[javac] where CAP#1 is a fresh type-variable:
[javac] CAP#1 extends Exception from capture of ? extends Exception
[javac] 1 error
It would seem to me that the according to the message all should be correct. CAP#1indeed extends Exception. So how should the above message be understood? SSCCE below (initially not posted since I was hoping to understand the error message itself in the general case):
在我看来，根据消息，一切都应该是正确的。CAP#1确实扩展了异常。那么应该如何理解上面的消息呢？下面的 SSCCE（最初没有发布，因为我希望在一般情况下理解错误消息本身）：
class Holder<T> {
public T t;
public Holder(T t) {
this.t = t;
}
}
public class FooMain {
public static void main(String args[]) throws Exception {
Holder<Class<? extends Exception>> exceptionClassHolder;
exceptionClassHolder = new Holder<>( (new Exception()).getClass() );
}
}
采纳答案by Paul Bellora
Unfortunately, the existing answers don't explain what's going on here. First, the solution is to simply specify the type argument to Holder:
不幸的是，现有的答案并没有解释这里发生了什么。首先，解决方案是简单地将类型参数指定为Holder：
Holder<Class<? extends Exception>> exceptionClassHolder;
exceptionClassHolder =
new Holder<Class<? extends Exception>>(new Exception().getClass());
The reason your version didn't work is because new Exception().getClass()returns a Class<? extends Exception>, where ?is a wildcard capture(referred to in the compiler error message as CAP#1). Since you use the "diamond operator" with new Holder<>, the compiler infers Class<CAP#1 extends Exception>for Tand so Holder<Class<CAP#1 extends Exception>>is the type of the created object.
您的版本不起作用的原因是new Exception().getClass()返回 a Class<? extends Exception>，其中?是通配符捕获（在编译器错误消息中称为CAP#1）。由于您使用的“钻石经营者”用new Holder<>，编译器推断Class<CAP#1 extends Exception>的T，因此 Holder<Class<CAP#1 extends Exception>>是创建的对象的类型。
However, this doesn't match your declared type of Holder<Class<? extends Exception>>. It uses a nested wildcard, which doesn't capture: while CAP#1 extends Exceptionis some specific typeextending Exception, the nested ? extends Exceptionrepresents literally any typeextending Exception.
但是，这与您声明的Holder<Class<? extends Exception>>. 它使用嵌套通配符，它不捕获：虽然CAP#1 extends Exception是某种特定类型的扩展Exception，但嵌套? extends Exception实际上表示任何类型的扩展Exception。
And while Class<CAP#1 extends Exception>is a subtype of Class<? extends Exception>, Holder<Class<CAP#1 extends Exception>>is not a subtype of Holder<Class<? extends Exception>>because generics aren't covariant, so the assignment fails.
而 whileClass<CAP#1 extends Exception>是 的子类型Class<? extends Exception>，Holder<Class<CAP#1 extends Exception>>不是 的子类型，Holder<Class<? extends Exception>>因为泛型不是协变的，所以赋值失败。
By manually specifying Class<? extends Exception>for T, you help the compiler avoid this "trap".
通过手动指定Class<? extends Exception>for T，您可以帮助编译器避免这个“陷阱”。
See my similar answers on these posts:
请参阅我在这些帖子上的类似答案：
回答by Joop Eggen
Holder<? extends Class<? extends Exception>> exceptionClassHolder;
exceptionClassHolder = new Holder<>( (new Exception()).getClass() );
The reason is that
原因是
- Because you are not using
Exception.classbut an Exception object, java thinks? extends Exceptionis required. - The same holds for
getClass(), again? extends Classneeded, though Class is final.
- 因为您使用的不是
Exception.classException 对象，所以 java 认为? extends Exception是必需的。 - 这同样适用于
getClass()，再次? extends Class需要，尽管 Class 是最终的。
Certainly one day this will be simplified.
当然有一天这会被简化。
