私有接口方法的方法参考 [英] Method reference to private interface method

问题描述

考虑以下代码：

public class A {
    public static void main(String[] args) {
        Runnable test1 = ((I)(new I() {}))::test;  // compiles OK
        Runnable test2 = ((new I() {}))::test;     // won't compile 
    }

    interface I {
        private void test() {}
    }
}

我真的不明白......我明白 test（）方法私有。但是，如果我们将匿名类强制转换为接口（（I）（new I（）{}）），会有什么变化？更准确地说，我希望看到一个允许该技巧的特定JLS点。

I don't really get the point... I understand that test() method is private. But what is changed if we cast an anonymous class to its interface ((I)(new I() {}))? More precisely I would like to see a particular JLS point which allows that trick.

PS 我已将其报告为编译器的错误（ID ：9052217）。在我看来， Runnable test2 =（（new I（）{}））:: test; 应该在这种特殊情况下编译好。

P.S. I have reported it as a bug of compiler (ID : 9052217). It seems to me that Runnable test2 = ((new I() {}))::test; should be compiled fine in this particular case.

PPS 到目前为止，根据我的报告创建了一个错误： https://bugs.openjdk.java.net/browse/JDK-8194998 。它可能会被关闭为无法修复或无论如何。

P.P.S. So far there was created a bug based on my report: https://bugs.openjdk.java.net/browse/JDK-8194998 . It might be that it will be closed as "won't fix" or whatsever.

推荐答案

这不是一个新问题，并且与私有接口方法或方法引用无关。

This is not a new issue, and has nothing to do with private interface methods or method references.

如果更改代码以扩展类而不是实现接口，并调用方法而不是引用它，你仍然得到完全相同的问题。

If you change code to extend a class instead of implement an interface, and to call the method instead of referencing it, you still get exact same problem.

class A {
    public static void main(String[] args) {
        ((I)(new I() {})).test();  // compiles OK
        ((new I() {})).test();     // won't compile 
    }

    class I {
        private void test() {}
    }
}

但是，该代码可以应用于较旧的Java版本，我尝试了Java 9,8,7,6,5和1.4 。所有行为都相同!!

However, that code can be applied to older Java versions, and I tried Java 9, 8, 7, 6, 5, and 1.4. All behave the same!!

问题是私有方法不是继承的 ¹ ，所以匿名类没有这个方法，一点都不由于私有方法甚至不存在于匿名类中，因此无法调用它。

The issue is that private methods are not inherited¹, so the anonymous class doesn't have the method, at all. Since the private method doesn't even exist in the anonymous class, it cannot be called.

当您转换为时我，现在该方法存在供编译器查看，并且由于 I 是一个内部类，因此您被授予访问权限（通过合成方法），即使它是私有的。

When you cast to I, the method now exists for the compiler to see, and since I is an inner class, you are granted access (through a synthetic method), even though it is private.

在我看来，这不是一个错误。这是私有方法在继承环境中的工作方式。

In my opinion, it is not a bug. It's how private methods work in context of inheritance.

^{1）as 由Jorn Vernee发现在 JLS 6.6-5 ：[私有类成员]不是由子类继承的。}

^{1) As found by Jorn Vernee in JLS 6.6-5: "[A private class member] is not inherited by subclasses".}

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