val and object inside a scala class?

Question

What is the difference between declaring a field as val, lazy val and object inside a scala class, as in the following snippet:

<

Answer 1

I'm not sure that aioobe recognized the significance of his answer, but the different types actually represent a critical difference between vals and objects. In particular, the val and lazy val have a structural type (e.g. A{def foo: Int}), while the object has a singleton type. As a result, calls to the foo method on the vals involve reflection, while calls to the foo method on the object do not:

class A

class B {
  val a1 = new A      { def foo = printStack }
  object a2 extends A { def foo = printStack }
  lazy val a3 = new A { def foo = printStack }

  def printStack() = 
     new Exception().getStackTrace take 3 foreach println
}

scala> val b = new B
b: B = B@5c750

scala> b.a1.foo   // the val
line124$object$$iw$$iw$B.printStack(<console>:12)
line124$object$$iw$$iw$B$$anon$1.foo(<console>:7)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

scala> b.a2.foo   // the object
line124$object$$iw$$iw$B.printStack(<console>:12)
line124$object$$iw$$iw$B$a2$.foo(<console>:8)
line128$object$$iw$$iw$.<init>(<console>:9)

scala> b.a3.foo   // the lazy val
line124$object$$iw$$iw$B.printStack(<console>:12)
line124$object$$iw$$iw$B$$anon$2.foo(<console>:9)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)