Abstract Methods in Scala Traits
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'
}
}