Parent Classes

In contrast to Java, all values in Scala are objects (including numerical values and functions). Since Scala is class-based, all values are instances of a class.

Class hierarchy is linear, a class can only extend from one parent class:

class Soldier(val firstName: String, val lastName: String) {}
class Pilot(override val firstName: String, override val lastName: String, val squadron: Long)
  extends Soldier(firstName, lastName)
val pilot = new Pilot("John", "Yossarian", 256)
pilot.firstName should be(res0)
pilot.lastName should be(res1)

A class that extends from another is polymorphic:

class Soldier(val firstName: String, val lastName: String) {}
class Pilot(override val firstName: String, override val lastName: String, val squadron: Long)
  extends Soldier(firstName, lastName)

val pilot = new Pilot("John", "Yossarian", 256)
val soldier: Soldier = pilot

soldier.firstName should be(res0)
soldier.lastName should be(res1)

An abstract class, as in Java, cannot be instantiated and only inherited:

abstract class Soldier(val firstName: String, val lastName: String) {}

// if you uncomment this line, it will fail compilation
//val soldier = new Soldier

A class can be placed inside an abstract class just like in Java:

abstract class Soldier(val firstName: String, val lastName: String) {

  class Catch(val number: Long) {
    // nothing to do here.  Just observe that it compiles
  }

}
class Pilot(override val firstName: String, override val lastName: String, val squadron: Long)
  extends Soldier(firstName, lastName)

val pilot = new Pilot("John", "Yossarian", 256)
val catchNo = new pilot.Catch(22) //using the pilot instance's path, create an catch object for it.
catchNo.number should be(res0)