Companion object cannot access private variable on the class

Question

A rather weird behavior coming from the Scala REPL.

Although the following compiles without a problem:

class CompanionObjectTest {
    private val x          


        

Answer 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._