// 最简版
fun main(args: Array<String>) {
  println("Hello, world!")
}

// 面向对象版
class Greeter(val name: String) {
  fun greet() {
    println("Hello, $name")
  }
}

fun main(args: Array<String>) {
  Greeter(args[0]).greet()
}

var answer = 12
answer = 13 // 编译通过

val age = 18
age = 19 // 编译错误

var a: String? = "abc"
a = null // 编译通过

val $name = 'world'
println("Hello, $name!")

fun main(args: Array<String>) {
  if (args.size > 0) {
    println("Hello, ${args[0]}!")
  }
  
  println("Hello, ${if (args.size > 0) args[0] else "someone"}!")
}

fun max(a: Int, b: Int): Int {
  return if (a > b) a else b
}

fun <T> joinToString(
  collection: Collection<T>,
  separator: String,
  prefix: String,
  postfix: String
): String {
  val result = StringBuilder(prefix)
  
  for ((index, element) in collection.withIndex()) {
    if (index > 0) result.append(separator)
    result.append(element)
  }
  
  result.append(postfix)
  return result.toString()
}

joinToString(collection, separator = " ", prefix = " ", postfix = ".")

>>> val list = listOf(1, 2, 3)
>>> println(list)
[1, 2, 3]

fun <T> joinToString(
  collection: Collection<T>,
  separator: String = ", ",
  prefix: String = "",
  postfix: String = ""
): String {
  val result = StringBuilder(prefix)
  
  for ((index, element) in collection.withIndex()) {
    if (index > 0) result.append(separator)
    result.append(element)
  }
  
  result.append(postfix)
  return result.toString()
}

>>> joinToString(list, ", ", "", "")
1, 2, 3
>>> joinToString(list)
1, 2, 3
>>> joinToString(list, "; ")
1; 2; 3

fun readNumber(reader: BufferedReader): Int? {
  try {
    val line = reader.readLine()
    return Integer.parseInt(line)
  }
  catch (e: NumberFormatException) {
    return null
  }
  finally {
    reader.close()
  }
}

enum class Color {
  RED, ORANGE, YELLOW, GREEN, BLUE, INDIGO, VIOLET
}