Question

Suppose I have the following abstract class:

abstract class A (var is_happy : Boolean) {
  def toggle_happiness();
}

And now I want to define a concrete class which implements the toggle_happiness() method:

class B (is_happy : Boolean) extends A (is_happy) {
  def toggle_happiness() = {
    is_happy = !is_happy
  }
}

Scala’s compiler gives me:

error: reassignment to val
   is_happy = !is_happy
            ^

What’s going on here? I thought that is_happy referred to a var in my class that is set by my constructor. Do I have a conflict with the name is_happy?

Thanks,
Dan

Answer 1

See this question. Essentially, Scala thinks that you’re trying to assign to the constructor parameter, is_happy, rather than the var, is_happy, which just happens to have the same name. Some solutions are to:

• Make the var abstract in the base class.
• Rename the constructor parameter (e.g. _is_happy). Since parameter names are part of the public API of your constructors/methods, this may not be advisable.

You’re fortunate that the problem was detected at compile time in your case. This issue can lead to very surprising runtime behavior when it goes undetected.