Question

Possible Duplicate:
val and object inside a scala class?

Is there a substantive difference between:

class Foo {
  object timestamp extends java.util.Date
}

and

class Foo {
  val timestamp = new java.util.Date {}
}

What does it really mean to have a class with an object field? What are they used for? Are there situations where you must use an object?

Thanks…

Answer 1

Using an object may be preferable if you need to add behavior to the field. For example:

class Foo {
   object startDate extends java.util.Date {
      def isBusinessDay: Boolean = // ...
   }
}

class Bar {
   lazy val startDate = new java.util.Date {
      def isBusinessDay: Boolean = // ...
   }
}

The type of foo.startDate is foo.startDate.type, and a call to the foo.startDate.isBusinessDay method will be resolved statically.

The type of bar.startDate, on the other hand, is the structural type java.util.Date{ def isBusinessDay: Boolean }. A call to bar.startDate.isBusinessDay will therefore use reflection and incur unnecessary runtime overhead.