CODEWITHSUNDEEP
scalainheritanceextendstraits
user1819676
user1819676

Reputation: 369

In scala is it possible for a trait to extend a class which needs parameters?

I know a trait can extend a class which has an empty parameter constructor:

class Foo
trait Bar extends Foo

but is it possible to extend a class which constructor has some parameters?

class Foo(b: Boolean)
trait Bar extends Foo(true)

Is it possible to achieve this? It seems it's not possible. but why?

Thanks

Upvotes: 11

Views: 424

Answers (2)

Alexey Romanov
Alexey Romanov

Reputation: 170745

Yes, it's possible, you just can't give constructor arguments:

trait Bar extends Foo { ... }

But to instantiate it you need to call the constructor as well:

new Foo(false) with Bar

Upvotes: 4

jwvh
jwvh

Reputation: 51271

Making the arg a val seems to work.

scala> class Foo(val b: Boolean)
defined class Foo

scala> trait Bar extends Foo {override val b = true}
defined trait Bar

This also works if you make the class a case class which automatically turns the args in to vals.

EDIT

As @Aleksey has pointed out, this compiles but it's a trait that can't be instantiated so, no, it still doesn't seem possible. You'll have to make Bar a class.

scala> class Bar extends Foo(false) {println(b)}
defined class Bar

scala> new Bar
false

Upvotes: 2

Related Questions