Question

I’m new to scala from java and confused by sequence of object initialization of scala in an inherent hierarchy. IIRC, in Java, if an object of sub-class is initialized, constructor of its base class is invoked before any code of its own constructor. While in scala, I get totally different behavior. Consider the following example:

class Point(val x: Int, val y: Int){
    val name = this.makeName;

    def makeName: String = {
        println("makeName at super.");

        "[" + x + ", " + y + "]";
    }

    override def toString: String = name;
}

class ColorPoint(override val x: Int, override val y: Int, var color: String) extends Point(x, y) {

    // key statement
    println(name);

    override def makeName: String = {
        println("makeName at sub.");

        super.makeName + ":" + myColor;
    }

    val myColor = color;

    override def toString: String = name;
}

Let’s just consider byte code of constructor of ColorPoint dumped with javap. If the code include the key statement println(name); the byte code is

public ColorPoint(int, int, java.lang.String);
  Code:
   0:   aload_0
   1:   aload_3
   2:   putfield        #13; //Field color:Ljava/lang/String;
   5:   aload_0
   6:   iload_1
   7:   iload_2
   8:   invokespecial   #18; //Method Point."<init>":(II)V
   11:  getstatic       #24; //Field scala/Predef$.MODULE$:Lscala/Predef$;
   14:  aload_0
   15:  invokevirtual   #28; //Method name:()Ljava/lang/String;
   18:  invokevirtual   #32; //Method scala/Predef$.println:(Ljava/lang/Object;)V
   21:  aload_0
   22:  aload_3
   23:  putfield        #34; //Field myColor:Ljava/lang/String;
   26:  return

We can see field myColor is initialized after invokespecial, i.e. after initialization of the base class.

If I comment out the statement println(name); the byte code is:

public ColorPoint(int, int, java.lang.String);
  Code:
   0:   aload_0
   1:   aload_3
   2:   putfield        #13; //Field color:Ljava/lang/String;
   5:   aload_0
   6:   aload_3
   7:   putfield        #15; //Field myColor:Ljava/lang/String;
   10:  aload_0
   11:  iload_1
   12:  iload_2
   13:  invokespecial   #20; //Method Point."<init>":(II)V
   16:  return

We see that field myColor is initialized just before invokespecial, i.e. before base is initialized.

Then what’s the reason? Any document/article specifies this kind of behavior?

BTW, version of my scala is 2.7.7final (OpenJDK Server VM, Java 1.6.0_20). Thanks and Best Regards!

Answer 1

The compiler is simply executing things in order. There is some documentation here.

https://github.com/paulp/scala-faq/wiki/Initialization-Order

The main part from it is the following.

1. Superclasses are fully initialized before subclasses.
2. Otherwise, in declaration order.