How can I create a read-only class member in Scala?

Question

I want to create a Scala class where one of its var is read-only from outside the class, but still a var. How can I do it?

If it was a val, there was no need to do anything. By default, the definition implies public access and read-only.

Answer 1

Define a public "getter" to a private var.

scala> class Foo {
     |   private var _bar = 0
     |
     |   def incBar() { 
     |     _bar += 1 
     |   }
     |
     |   def bar = _bar
     | }
defined class Foo

scala> val foo = new Foo
foo: Foo = Foo@1ff83a9

scala> foo.bar
res0: Int = 0

scala> foo.incBar()

scala> foo.bar
res2: Int = 1

scala> foo.bar = 4
<console>:7: error: value bar_= is not a member of Foo
       foo.bar = 4
           ^

Answer 2

Define a trait with the "getter" method:

trait Foo { def bar: T }

Define a class which extends this trait, and which has your variable

private class FooImpl (var bar: T) extends Foo

Restrict the visibility of this class appropriately.

Having a dedicated interface allows you also to use multiple implementation classes at runtime, e.g. to cover special cases more efficiently, lazy loading etc.