renaudcerrato

val listener = object: OnMenuClickListener, OnMenuExpandListener {
  override fun onMenuClick(item: Menu)  { ... }
  override fun onMenuExpand(item: Menu) { ... }
}

menu.setOnClickListener(listener)
menu.setOnExpandListener(listener)

renaudcerrato

class Person private constructor(val name: String, val age: Int) {
  
  companion object Factory {
    fun create(name: String, age: Int) = Person(name, age)
  }
}

// members of the companion object can be called using the class name as qualifier
val bob = Person.create("Bob", 21)

renaudcerrato

// object class as a singleton
object Log {
  val prop = 42
  fun debug(msg: String) { ... }
  fun info(msg: String) { ... }
  fun warn(msg: String) { ... }
  fun error(msg: String) { ... }
}

// object classes can inherit from classes and interfaces

renaudcerrato

class A {
  // nested class (equivalent to static in Java)
  class B {
    // A can't see private members of nested classes
    private val invisible = 42
  }
  
  // inner class
  inner class C {
    // A can't see private members of inner classes

renaudcerrato

class Point(val x: Double, val y: Double) {
  // addition operator
  operator fun plus(other: Point): Point {
    return Point(x + other.x, y + other.y)
  }
  
  // overload using Double
  operator fun plus(value: Double): Point {
    return Point(x + value, y + value)
  }

renaudcerrato

interface View {
  fun click()
  fun toggle()
}

class ViewImpl: View {
  override fun click()  { println("click!") }
  override fun toggle() { println("toggle!") }
}

renaudcerrato

interface A {
  val a: Int     // abstract
  val b: Int     // abstract 
  fun foo()      // abstract
  fun bar() {
    // optionnal body
  }
}

interface B { ... }

renaudcerrato

// final public class (default)
class A

// final private class (only visible in the file)
private class B

// final internal class (only visible in the module)
internal class C

// abstract public class

renaudcerrato

class DeepThought {
  val answer: String by lazy {
    sleep(7_500_000, ChronoUnit.YEARS)
    "42"
  }
}

renaudcerrato

class Presenter {
  lateinit var view: View
  
  fun setup(_view: View) {
    // ensure the lateinit var hasn't been initialized
    if(this::view.isInitialized) throw IllegalStateException()
    view = _view
  }
}