伴生对象无法访问类上的私有变量

发布于 2024-11-28 02:48:23 字数 728 浏览 0 评论 0原文

来自 Scala REPL 的相当奇怪的行为。

尽管以下编译没有问题：

class CompanionObjectTest {
    private val x = 3
}
object CompanionObjectTest {
    def testMethod(y:CompanionObjectTest) = y.x + 3
}

私有变量似乎无法从 REPL 中的伴生对象访问：

scala> class CompanionObjectTest {
     | 
     | private val x = 3;
     | }
defined class CompanionObjectTest

scala> object CompanionObjectTest {
     | 
     | def testMethod(y:CompanionObjectTest) = y.x + 3
     | }
<console>:9: error: value x in class CompanionObjectTest cannot be accessed in CompanionObjectTest
       def testMethod(y:CompanionObjectTest) = y.x + 3
                                                 ^

为什么会发生这种情况？

原文

A rather weird behavior coming from the Scala REPL.

Although the following compiles without a problem:

class CompanionObjectTest {
    private val x = 3
}
object CompanionObjectTest {
    def testMethod(y:CompanionObjectTest) = y.x + 3
}

the private variable does not seem to be accessible from the companion object in REPL:

scala> class CompanionObjectTest {
     | 
     | private val x = 3;
     | }
defined class CompanionObjectTest

scala> object CompanionObjectTest {
     | 
     | def testMethod(y:CompanionObjectTest) = y.x + 3
     | }
<console>:9: error: value x in class CompanionObjectTest cannot be accessed in CompanionObjectTest
       def testMethod(y:CompanionObjectTest) = y.x + 3
                                                 ^

Why is that happening?

分享到QQ

分享到微博

如果你对这篇内容有疑问，欢迎到本站社区发帖提问参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。

发布评论

需要登录才能够评论，你可以免费注册一个本站的账号。

爱要勇敢去追 2024-12-05 02:48:23

所发生的情况是，REPL 上的每条“行”实际上都放置在不同的包中，因此类和对象不会成为同伴。您可以通过多种方式解决此问题：

进行链类和对象定义：

scala> class CompanionObjectTest {
     |   private val x = 3;
     | }; object CompanionObjectTest {
     |   def testMethod(y:CompanionObjectTest) = y.x + 3
     | }
defined class CompanionObjectTest
defined module CompanionObjectTest

使用粘贴模式：

scala> :paste
// Entering paste mode (ctrl-D to finish)

class CompanionObjectTest {
    private val x = 3
}
object CompanionObjectTest {
    def testMethod(y:CompanionObjectTest) = y.x + 3
}

// Exiting paste mode, now interpreting.

defined class CompanionObjectTest
defined module CompanionObjectTest

将所有内容放入对象内：

scala> object T {
     | class CompanionObjectTest {
     |     private val x = 3
     | }
     | object CompanionObjectTest {
     |     def testMethod(y:CompanionObjectTest) = y.x + 3
     | }
     | }
defined module T

scala> import T._
import T._

What is happening is that each "line" on REPL is actually placed in a different package, so the class and the object do not become companions. You can solve this in a few ways:

Make chain class and object definitions:

scala> class CompanionObjectTest {
     |   private val x = 3;
     | }; object CompanionObjectTest {
     |   def testMethod(y:CompanionObjectTest) = y.x + 3
     | }
defined class CompanionObjectTest
defined module CompanionObjectTest

Use paste mode:

scala> :paste
// Entering paste mode (ctrl-D to finish)

class CompanionObjectTest {
    private val x = 3
}
object CompanionObjectTest {
    def testMethod(y:CompanionObjectTest) = y.x + 3
}

// Exiting paste mode, now interpreting.

defined class CompanionObjectTest
defined module CompanionObjectTest

Put everything inside an object:

scala> object T {
     | class CompanionObjectTest {
     |     private val x = 3
     | }
     | object CompanionObjectTest {
     |     def testMethod(y:CompanionObjectTest) = y.x + 3
     | }
     | }
defined module T

scala> import T._
import T._

回复收藏 0 原文

往事风中埋 2024-12-05 02:48:23

这确实有点奇怪。要解决此问题，您应该首先使用 :paste 进入粘贴模式，然后定义您的类和伴生对象，并使用 CTRL-D 退出粘贴模式。下面是一个 REPL 会话示例：

Welcome to Scala version 2.9.0.1 (OpenJDK Server VM, Java 1.6.0_22).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :paste
// Entering paste mode (ctrl-D to finish)

class A { private val x = 0 }
object A { def foo = (new A).x }

// Exiting paste mode, now interpreting.

defined class A
defined module A

scala>

This is indeed a little weird. To work around this problem, you should enter paste mode first with :paste, then define your class and your companion object and exit paste mode with CTRL-D. Here is a sample REPL session:

Welcome to Scala version 2.9.0.1 (OpenJDK Server VM, Java 1.6.0_22).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :paste
// Entering paste mode (ctrl-D to finish)

class A { private val x = 0 }
object A { def foo = (new A).x }

// Exiting paste mode, now interpreting.

defined class A
defined module A

scala>

回复收藏 0 原文

~没有更多了~