Class hierarchy

abstract class

Abstract class can leave the methods unimplemented. However, it can't be instanced.

Superclass vs Subclass

Subclass extends superclass.

If no superclass is given, Object from java.lang is default superclass.

The direct or indirect superclass of C is call the base class of C.

Object

Object defines a singleton, and it's a value so that:

• There's no other instances.

• It doesn't take variables.

object Person {
  val N_EYES = 2
}

val mary = Person
val paul = Person
println(mary == paul) // true

Companion class/object

An object and a class can have the same name.

Object plays the role similar to static class definition.

object Person {
  // factory method
  def apply(mother: Person, father: Person): Person = new Person(mother.name + father.name)
}
class Person(val name: String)

val mother = new Person("Mary")
val father = new Person("Jack")
val kid = Person(mother, father)

Programs

We can create an Object that contains main function, and it will be acted as a program.

Object Hello {
  def main(args: Array[string]): Unit = println("Hello, world")
}

> scala Hello

Another way is using @main to create a function.

@main def birthday(name: String, age: Int) = {
  println(s"Happy birthday, $name! $age years old.")
}

> scala birthday Peter 11