dodyg/gist:5616605

## gistfile1.md

      
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    This is a quick guide to Kotlin programming language. The previous part of this guide is here
#Object Oriented
fun main(args : Array<String>) {
  class local (val x : Int)
  
  val y = local(10)
  println("${y.x}")
}
Above code is a sample of Local Class, one of many support that Kotlin has for OO programming.

Abstract classes
Primary constructor
Delegation
Generic classes
Class objects
Nested classes
Local classes
Object expressions
Traits
Data classes
Anonymous Analyzer
Anonymous Objects

##Kotlin classes
Kotlin classes does not have:

Static member (methods or properties)
Secondary constructors
No fields, just properties

###Simplest Kotlin class definition
class Person


fun main(args : Array<String>) {
  val p = Person()
  val name = javaClass<Person>().getSimpleName()
  println("$name")
}
The class Person is as simple as you can get to declare a class
by default, a Kotlin class is final. So to make a class inheritable, you must you the keyword open in front of it
open class Person
class Hero : Person()


fun main(args : Array<String>) {
  val name = javaClass<Person>().getSimpleName()
  println("$name")
  
  val name2 = javaClass<Hero>().getSimpleName()
  println("$name2")
}
Visibilities

Kotlin has four visibilities:

private
protected
internal
public

If you do not declare a visibility modifier, it is assumed to be internal visibility.
fun main(args : Array<String>) {
    val x = Visibility()
}

class Visibility{
 public var name : String = ""
 private var age : Int = 0
 protected var address : String = ""
 internal var friends : String = ""  
 var status : String = "Single"
}
An empty class is off course useless. Let's add some properties to it so it can hold data
open class Person
  
class Hero : Person(){
  public var name : String = ""
  public var age : Int = 30
  }


fun main(args : Array<String>) {
  val h = Hero()
  h.name = "Superman"
  h.age = 30
  
  println("${h.name} is ${h.age} years old")
}
Rule

Every declared property must be initialized, without exception.
a var property means it can be modified
a val property is a constant

##Primary constructor##
Unlike many other OO language, Kotlin only allows one single constructor
open class Person
  
class Hero (n: String, a : Int) : Person(){
  public var name : String = n
  public var age : Int = a
  }


fun main(args : Array<String>) {
  val h = Hero("Superman", 30)
  println("${h.name} is ${h.age} years old")
}
As you can see, the constructor parameter n and a are being used to initialized their respective properties.