meaning of top level private class in scala

Question

Say in a scala file I define a class like

private class Test{}

what does the private here mean? In java you can't have a top level private class, which obviously makes sense. But private is syntactically valid in scala, hope someone can shed some light upon it.

Answer 1

You could specify package name for private modifier to allow access to this class only from specified package. By default (without package specified) it is visible only for other members in the enclosing package.

$ cat > test.scala <<EOF
package myPackage {
  private[myPackage] class Test
  private object A extends Test
}

package otherPackage {
  object B extends myPackage.Test
}
EOF

$ scalac test.scala 
test.scala:7: error: class Test in package myPackage cannot be accessed in package myPackage
  object B extends myPackage.Test
                             ^
one error found

For instance you can access private class from its companion object like this:

trait ITest
private class Test extends ITest
object Test {
  def apply(): ITest = new Test
}

Test()
// ITest = Test@59e2abc3 

Further clarification on examples:

package myPackage {
  private class Test
  private object A extends Test
  object B extends myPackage.Test //Compile error: private class Test escapes its defining scope as part of type myPackage.Test
  private object C extends myPackage.Test // works since C is also private

  object Test {
     def apply() = new Test //error: private class Test escapes its defining scope as part of type myPackage.Test
   } 
   object Test2 {
      def apply(): ITest = new Test //works as ITest is public 
    }
}

As long as instances of a private class do not escape the enclosing package scope it can be used within the package hierarchy. Companion objects accessing private classes has to be in same package hierarchy as well. With private[P] - p can be any package name that exists.