Traits

A trait is a reusable interface-plus-behavior unit. Declare it with trait Named { def name: String }. Traits can hold abstract members (no body) and concrete methods.

A class mixes in a trait with extends: class Person(val name: String) extends Named. Mix additional traits with with: extends Named with Greetable.

When the first parent is a trait (no class superclass), write extends TraitA with TraitB. Order matters for linearization when methods override each other — later mixins win for the same method name unless you use super.

Traits may take constructor parameters in modern Scala 3; in classic Scala 2 style (this track's stills), keep trait fields as abstract def/val members the class supplies.

Misses: writing interface / protocol keywords, using implements like Java, or treating a trait as a singleton object you never extend.

trait Named {
  def name: String
}

trait Greetable extends Named {
  def greet(): String = s"hi $name"
}

class Person(val name: String) extends Greetable

object TraitsDemo {
  def main(args: Array[String]): Unit = {
    val ada = new Person("Ada")
    println(ada.greet())
  }
}