Getting error when instantiating a trait within a class (method became private)

Question

In the following code, I'm trying to instantiate the trait A with the Decorator class and add a method p to it so I can get another object A but with a p method:

trait A {
  def x: Int
}

case class Decorator(a: A) {
  def withPrint: A = new A {
    val x = 100
    def p: Unit = println(x)
  }
}

val a = new A {val x = 100}

val d = Decorator(a).withPrint

But when I try to call d.p it gives error

value p is not a member of A$A54.this.A

Also then I print the declared methods in d I see p is private!

d.getClass.getDeclaredMethods.foreach(println)

# public int stringsimilarity.A$A55$A$A55$Decorator$$anon$2.x()
# private void stringsimilarity.A$A55$A$A55$Decorator$$anon$2.p()

Could anyone explain why this happens? Any help will be really appreciated!! Thank you!

Andrey Tyukin · Accepted Answer · 2018-07-21T11:44:38.020

1

There is no reason why def p should be public. It's a method that you defined in a local anonymous class that implements trait A. The trait A declares only public method x, and nothing else. Therefore, the only scope where you have access to p is inside of the initializer of the new A { ... }, so private seems appropriate for that.

If you want to expose the method p, you can either add it to A (then you have to implement it there, and then you also don't need the withPrint method at all), or mix in an additional trait that guarantees the presence of method p. For example:

trait A {
  def x: Int
}

trait Print {
  def p: Unit
}

case class Decorator(a: A) {
  def withPrint: A with Print = new A with Print {
    val x = 100
    def p: Unit = println(x)
  }
}

val a = new A { val x = 100 }

val d = Decorator(a).withPrint

d.p

edited Jul 21 '18 at 11:44

answered Jul 20 '18 at 20:40

Andrey Tyukin

43,673
4
57
93

1

Thank you Andrey! Using mixin makes a lot more sense, you are the man! – Bubble Bubble Bubble Gut Jul 20 '18 at 20:42
Does it work if we add the `def p: Unit` abstract method directly in `trait A` by removing the `trait Print`completely and also the `with Print` mixin in `withPrint` method.? It is working if we add the `implemented method p` in trait A but not with abstract method..can you please clarify this Andrey Tyukin? – RAGHHURAAMM Jul 21 '18 at 10:48
@RAGHHURAAMM If we add `def p: Unit` to `trait A`, then we have to provide it by the time we instantiate `val a = new A { ... }`, that is, we either have to include it in `trait A { ... }` right from the beginning, or add it to each `new A { ... }`. In both cases, `withPrint` method as such becomes useless. So, if you wanted to keep `withPrint` that does something halfway-meaningful, I don't see how to avoid mixing in or refining the return type. – Andrey Tyukin Jul 21 '18 at 11:47

Getting error when instantiating a trait within a class (method became private)

1 Answers1