对Object.clone（）的功能引用不会编译 [英] Functional reference to Object.clone() doesn't compile

问题描述

编译

  import java .util.concurrent.Callable; 
 
 class Ideone 
 {
 Callable<？> x = super :: clone; 
} 
  

使用Oracle JDK提供：

  Main.java:6：错误：不兼容类型：无效方法引用
可调用<？> x = super :: clone; 
 ^ 
 clone（）在Object 
  

中具有受保护的访问权限因为类应该能够访问其父的受保护方法。这个表达式在Eclipse的编译器中工作良好。

此外，（）这是一个错误吗？

h2_lin>解决方案

super 实际上不是一个表达式，没有静态类型。 super.foo（）具有与 this.foo（）相同的访问权限;这就是说，方法调用在字节代码中以不同的方式翻译为超级调用，而不是正常调用。

JLS在此不是​​很清楚;例如在受保护的访问部分中， a>，未提及 super.protectedMember 表单;但显然这种形式应该在JLS中讨论;它应该是可访问的。 （该部分确实建议 X :: m 和 Xm 应该被视为相同的wrt访问权限）

在方法参考，措辞也是模糊的;但是， super :: clone 应该可以访问 super.clone（） >

已创建一个错误报告： JDK- 8139836：当x受保护时不能使用super :: x方法引用。它的当前状态是它将在Java 9中修复。

Compiling

import java.util.concurrent.Callable;

class Ideone
{
    Callable<?> x = super::clone;
}

using Oracle JDK gives:

Main.java:6: error: incompatible types: invalid method reference
    Callable<?> x = super::clone;
                    ^
    clone() has protected access in Object

which makes no sense as a class should be able to access its parent's protected methods. This expression works fine in Eclipse's compiler.

Also, () -> super.clone() compiles fine....

Is this a bug?

解决方案

super is not actually an expression, and there's no static type to talk about. super.foo() has the same access as this.foo(); it's just that, the method invocation is translated differently in byte code, as "super invoke", as opposed to "normal invoke".

JLS is not very clear on this; e.g. in section of protected access, the super.protectedMember form is not mentioned; but obviously that form should be discussed in JLS; and it should be accessible. (The section does suggest that X::m and X.m should be treated the same w.r.t. access right)

In the section of method reference, the wording is also vague; nevertheless, super::clone should be accessible the same ways as super.clone() is accessible.

A bug report has been created: JDK-8139836: "Can't use super::x method reference when x is protected". Its current status is that it will be fixed in Java 9.

这篇关于对Object.clone（）的功能引用不会编译的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！