Constructor with non-instance variable assistant?

Question

I have a number of classes that look like this:

class Foo(val:BasicData) extends Bar(val) {
    val helper = new Helper(val)
    val derived1 = helper.getDerived1Value()
    val derived2 = helper.getDerived2Value()
}

...except that I don't want to hold onto an instance of "helper" beyond the end of the constructor. In Java, I'd do something like this:

public class Foo {
  final Derived derived1, derived2;
  public Foo(BasicData val) {
     super(val);
     Helper helper = new Helper(val);
     derived1 = helper.getDerived1Value();
     derived2 = helper.getDerived2Value();
  }
}

So how do I do something like that in Scala? I'm aware of creating a helper object of the same name of the class with an apply method: I was hoping for something slightly more succinct.

Answer 1

You could use a block to create a temporary helper val and return a tuple, like this:

class Foo(v: BasicData) extends Bar(v) {
  val (derived1, derived2) = {
    val helper = new Helper(v)
    (helper.getDerived1Value(), helper.getDerived2Value())
  }
}

Answer 2

Better look at the javap output (including private members) before you conclude this has side-stepped any fields for the Tuple2 used in the intermediate pattern-matching.

As of Scala 2.8.0.RC2, this Scala code (fleshed out to compile):

class BasicData
{
  def basic1: Int = 23
  def basic2: String = "boo!"
}

class Helper(v: BasicData)
{
  def derived1: Int = v.basic1 + 19
  def derived2: String = v.basic2 * 2
}

class Bar(val v: BasicData)

class   Foo(v: BasicData)
extends Bar(v)
{
  val (derived1, derived2) = {
    val helper = new Helper(v)
    (helper.derived1, helper.derived2)
  }
}

Produces this Foo class:

% javap -private Foo
public class Foo extends Bar implements scala.ScalaObject{
    private final scala.Tuple2 x$1;
    private final int derived1;
    private final java.lang.String derived2;
    public int derived1();
    public java.lang.String derived2();
    public Foo(BasicData);
}