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:

class A  class B {   val a1 = new A      { def foo = 1 }   object a2 extends A { def foo = 1 }   lazy val a3 = new A { def foo = 1 } } 

Answer 1

In the former, any code included is executed as soon as class B is created. In the latter, however, until you actually use the object, it won't be instantiated.

You can see the difference here:

class A { println("Creating a new A") } class B {   val a1 = new A { println("a1"); def foo = 1 }   object a2 extends A { println("a2"); def foo = 1 } }  scala> val b = new B Creating a new A a1 b: B = B@1176e8a  scala> b.a2.foo Creating a new A a2 res0: Int = 1 

There are also hidden differences in what the created .class files are named and such; and of course the two have different types.

Answer 2

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)