Sam Stites

Abstract Methods in Scala Traits

02 April 2015 - Reykjavik, Iceland

When you want to override an abstract method in a traits, you must declare both abstract on top of our override in order for this to work:

trait Logger {
  def log(msg: String)  // abstract

trait WarnLogger {
  abstract override def log(msg: String){
    super.log(s"[WARN] $msg")

If you fail to add the abstract modifier, Scala will think that the log method is concrete, will find an abstract log instead, and the compiler will error.

A trait can also have methods have abstract dependencies. One instance of this is the scala Iterator trait which has a ton of dependencies on the abstract next and hasNext methods.

An example with our Logger:

trait Logger {
  def log(msg: String)
  def info(msg: String)  { log(s"[INFO] $msg")  }
  def warn(msg: String)  { log(s"[WARN] $msg")  }
  def error(msg: String) { log(s"[ERROR] $msg") }

Which we can use like so:

class Foo with Logger { override def log(msg:String) { println(msg); }

def bar(msg: String) {
  error("Trouble ahead") // logs: '[ERROR] Trouble ahead'