Companion object cannot access private variable on

问题:

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?

回答1:

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._

回答2:

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>