Validator.kt

abstract class Validator() {
  var errorListener: OnErrorListener? = null
  var validateValue: Any? = null
  
  protected abstract fun hasError(): Boolean
  
  fun validate(): Boolean {
    if (hasError()) {
      errorListener?.onError()
      return false
    }
    return true
  }
  
  interface OnErrorListener {
      fun onError()
  }
}

class Main {
  val validator = ExtentedValidator()
  
  fun testValidator() {
    validator.errorListener = object : ValidationRule.OnErrorListener {
      override fun onError() {
        // show error
      }
    }
    
    if(validator.validate()) {
      // Do something ...
    }
  }
}

Try to use Lambda instead of listener interface.

var errorListener: (()->Unit)? = null

and change calling statement to

errorListener?.invoke()

Now in Main class, we can change to

validator.errorListener = {
  //show error
}