Scala Lazy Val Question

Question

I have a scenario where I have some objects that need to take in references from each other. The only way I can get this to compile is to use lazy

class A(b:B)
class B(a:A)
lazy val a:A = new A(b)
lazy val b:B = new B(a)

I can do the same thing using some actors, and get it to compile also

    abstract class Message
    case class Message1 extends Message
    case class Message2 extends Message

    class Actor1(otherActor:Actor) extends Actor {
        def act() {
            loop {
                react {
                    case Message1 =>
                        println("received message1")
                        otherActor ! Message2
                    case _ =>
                }
            }
        }
    }

    class Actor2(otherActor:Actor) extends Actor {
        def act() {
            loop {
                react {
                    case Message2 =>
                        println("received message2")
                        otherActor ! Message1
                    case _ =>
                }
            }
        }
    }

    lazy val actor1:Actor = new Actor1(actor2)
    lazy val actor2:Actor = new Actor2(actor1)

However, when I add the following:

    actor1.start
    actor2.start
    actor1 ! Message1

I get the following error:

Exception in thread "main" java.lang.NoClassDefFoundError: com/fictitiousCompany/stackOverflowQuestion/Test Caused by: java.lang.ClassNotFoundException: com.fictitiousCompany.stackOverflowQuestion.Test at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:307) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:248)

I'm using the Scala Eclipse Plugin 2.8.1.

Answer 1

Note that even your smaller example would have issues (in the REPL):

{
class A(b:B)
class B(a:A)
lazy val a:A = new A(b)
lazy val b:B = new B(a)
a
}
// causes stack overflow error

As soon as a needs to be evaluated therefore constructed, it would require B, which requires A. In order for this to work a or b would have to finish being constructed.

Using by-name parameters allows the smaller example to evaluate.

{
class A(b: => B)
class B(a: => A)
lazy val a:A = new A(b)
lazy val b:B = new B(a)
a
}

Note sure if that'll work for your actor example as well.

Edit: by name params worked locally on 2.8.0. I replaced case class with object to get rid of some deprecation warnings and added start methods on actor1, actor2 and kick the whole thing with actor1 ! Message1. Aside from this I haven't used actor before, so I can't comment more. Here is what I tested:

import scala.actors._

abstract class Message
object Message1 extends Message
object Message2 extends Message

class Actor1(otherActor: => Actor) extends Actor {
def act() {
    loop {
    react {
        case Message1 =>
        println("received message1")
        otherActor ! Message2
        case _ =>
    }
    }
}
}

class Actor2(otherActor: => Actor) extends Actor {
def act() {
    loop {
    react {
        case Message2 =>
        println("received message2")
        otherActor ! Message1
        case _ =>
    }
    }
}
}

{
  lazy val actor1:Actor = new Actor1(actor2)
  lazy val actor2:Actor = new Actor2(actor1)
  actor1.start
  actor2.start
  actor1 ! Message1
}

Prints a bunch of:

received message1
received message2