vals and inheritance

gistfile1.scala

abstract class Foo {
  val a: String
  val b = "hello" + a
}

class Bar extends Foo {
  val a = "world"
}

// What is wrong with the above code? 

// The superclass constructor is invoked before the subclass constructor. As a 
// result the value of 'a' is uninitialized (null) when we are initializing
//  the value of b