In these cases, the Scala value class will be “boxed”, right?

Question

If I have this value class:

class ActionId(val value: Int) extends AnyVal

Then, in all the examples below, an object will be allocated for

Answer 1

In all three cases there will be no boxing at all.

It's quite easy to check by your self:

class ActionId(val value: Int) extends AnyVal
object Foo {
  def someFunction(): ActionId = {
    new ActionId(123)
  }
}

Now lets run scalac, and we will have a bunch of class files (files with bytecode). The one that we need is Foo\$.

» javap Foo\$ 
Compiled from "Boxing.scala"
public final class Foo$ extends java.lang.Object{
    public static final Foo$ MODULE$;
    public static {};
    public int someFunction();
}

As you can see, even if value class leaks from function generally it wouldn't be boxed.

case class Post(id: ActionId, notion: String)

object Foo2 {
  def someFunction(): Post = {
    Post(new ActionId(123), "So ActionID will be boxed?")
  } 
}

scalac => javap:

» javap Post  
Compiled from "Boxing.scala"
public class Post extends java.lang.Object implements scala.Product,scala.Serializable{
    public static scala.Function1 tupled();
    public static scala.Function1 curried();
    public int id();
    public java.lang.String notion();
    public Post copy(int, java.lang.String);
    public int copy$default$1();
    public java.lang.String copy$default$2();
    public java.lang.String productPrefix();
    public int productArity();
    public java.lang.Object productElement(int);
    public scala.collection.Iterator productIterator();
    public boolean canEqual(java.lang.Object);
    public int hashCode();
    public java.lang.String toString();
    public boolean equals(java.lang.Object);
    public Post(int, java.lang.String);
}

As you can see id here represented as plain int (e.g. third method).

Finally, will value class boxed if the object with a value class member is not returned (doesn't really escape the scope)?

case class Post(id: ActionId, notion: String)

object Foo3 {
  def anotherFunction(): Unit {
    val post = Post(new ActionId(123), "Will be boxed?")
  } 
}

If we look closely at bytecode of the method, here is what we will see:

Code:
   Stack=4, Locals=2, Args_size=1
   0:   new #15; //class Post
   3:   dup
   4:   bipush  123
   6:   ldc #17; //String Will be boxed?
   8:   invokespecial   #20; //Method Post."":(ILjava/lang/String;)V
   11:  astore_1
   12:  return
  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      13      0    this       LFoo3$;
   12      0      1    post       LPost;

There is no boxing of int in ActionId. If it would box you would see something like this one:

Code:
   Stack=5, Locals=2, Args_size=1
   0:   new #15; //class Post
   3:   dup
   4:   new #17; //class ActionId
   7:   dup
   8:   bipush  123
   10:  invokespecial   #20; //Method ActionId."":(I)V
   13:  ldc #22; //String Will be boxed?
   15:  invokespecial   #25; //Method Post."":(LActionId;Ljava/lang/String;)V
   18:  astore_1
   19:  return
  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      20      0    this       LFoo3$;
   19      0      1    post       LPost;

You see, the difference is bipush 123 versus

   4:   new #17; //class ActionId
   7:   dup
   8:   bipush  123
   10:  invokespecial   #20; //Method ActionId."":(I)V