静态嵌套类作为泛型类型绑定不起作用 [英] Static nested class as generic type bound doesn't work

问题描述

文件 AbstractContainer.java

 包容器; 
 
 import static container.AbstractContainer。*; 
 
公共抽象类AbstractContainer< ElementType extends AbstractElement> {
 public static abstract class AbstractElement {
} 
} 
  

文件 ConcreteElement.java

 包容器; 
 
导入静态容器.ConcreteContainer。*; 
 import static container.AbstractContainer。*; 
 
 public class ConcreteContainer extends AbstractContainer< ConcreteElement> {
 public static class ConcreteElement extends AbstractElement {
} 
} 
  

这段代码给了我一个编译错误：

  java：type参数container.ConcreteContainer.ConcreteElement不在类型变量的范围内ElementType 
  

但IDE没有发现任何问题（IDEA 12）。

第一：这是怎么回事？第二个问题，在 AbstractContainer.java 中，为什么我有静态导入嵌套类，这显然是在范围内，以泛型类型使用它（ extends AbstractElement 而不是 extends AbstractContainer.AbstractElement
$ b $

解决方案

第二个问题 - 可能是由于技术性原因：



ht tp：//docs.oracle.com/javase/specs/jls/se7/html/jls-6.html#jls-6.3

在类类型C（§8.1.6）中声明或继承的成员 m 的声明范围是C <例如
$ b

 @SomeAnnotation（ M.class）
 class C 
< T扩展M> 
 {
 static class M {} 
} 
  

代码是非法的，因为 M 在 C 的主体之外，注解和类型参数中使用。

在Java 5之前，没有注释/泛型，因此C的整个主体涵盖了所有地方，M可以被明智地引用。但是现在这个规则已经过时了，我们应该真的扩大 M 的范围。我看不出有任何问题。

File AbstractContainer.java

package container;

import static container.AbstractContainer.*;

public abstract class AbstractContainer<ElementType extends AbstractElement> {
    public static abstract class AbstractElement {
    }
}

File ConcreteElement.java

package container;

import static container.ConcreteContainer.*;
import static container.AbstractContainer.*;

public class ConcreteContainer extends AbstractContainer<ConcreteElement> {
    public static class ConcreteElement extends AbstractElement {
    }
}

This code gives me a compile error:

java: type argument container.ConcreteContainer.ConcreteElement is not within bounds of type-variable ElementType

but the IDE doesn't see any problems (IDEA 12).

First: What is going on here?

Second question, In AbstractContainer.java why do I have to static import the nested class, that's obviously in scope, to use it in the generic type (extends AbstractElement instead of extends AbstractContainer.AbstractElement) ?

解决方案

First question - probably a compiler bug.

Second question - probably due to technicality:

http://docs.oracle.com/javase/specs/jls/se7/html/jls-6.html#jls-6.3

The scope of a declaration of a member m declared in or inherited by a class type C (§8.1.6) is the entire body of C

For example

@SomeAnnotation(M.class)
class C
<T extends M>
{
    static class M{}
}

the code is illegal, because M is used outside the body of C, in annotation, and in type parameter.

Prior to Java 5, there's no annotation/generics, therefore "the entire body of C" covers all places "M" can be sensibly referenced. But now the rule is outdated, we should really extend the scope of M a little bit; I don't see any problem in doing that.

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