// 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的内容来自变量。它可以随时改变。
var answer = 12
answer = 13 // 编译通过

// val的内容来自值。初始化之后不能改变。
val age = 18
age = 19 // 编译错误

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

var b: String = "abc"
b = null // 编译错误

// 整数类型
val answer: Int = 42
// 1.1版本开始支持类似swift的下划线分隔
val oneMillion = 1_000_000

// 浮点类型
val yearsToCompute = 7.5e6

// 字符类型
val question = "The Ultimate Question of Life, the Universe, and Everything"

// 字符串模板
val $name = 'world'
println("Hello, $name!")

// 大括号内可以是kotlin表达式
fun main(args: Array<String>) {
  if (args.size > 0) {
    println("Hello, ${args[0]}!")
  }
  
  println("Hello, ${if (args.size > 0) args[0] else "someone"}!")
}

// 常规定义
fun max(a: Int, b: Int): Int {
  return if (a > b) a else b
}

// 把函数当成表达式
fun max(a: Int, b: Int): Int = if (a > b) a else b

// 使用命名参数
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 = ".")

// 使用参数默认值（实现类似于重载的功能）
>>> 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

package strings

fun String.lastChar(): Char = this.get(this.length - 1)

// 定义一个类
class Person(val name: String)

// 定义类属性
class Person(
  val name: String,
  var isMarried: Boolean
)

// 定义类方法
class Person(
  val name: String,
  var isMarried: Boolean
) {
  fun sayHello() {
    println("hello $name")
  }
}

// 创建类实例
>>> val person = Person("Bob", true)
>>> println(person.name)
Bob
>>> person.sayHello()
hello BOb

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

// 定义接口
interface Clickable {
  fun click()
  fun showOff() = println("I'm clickable!") // 接口可以有默认的实现
}

// 实现接口
class Button : Clickable {
  override fun click() = println("I was clicked")
}

// 定义对象
object Payroll {
  val allEmployees = arrayListOf<Person>()
  
  fun calculateSalary() {
    for (person in allEmployees) {
      ...
    }
  }
}

// 使用对象
Payroll.allEmployees.add(Person(...))
Payroll.calculateSalary()

// 定义伴生对象
class A {
  companion object {
    fun bar() {
      println("Companion object called")
    }
  }
}

// 调用伴生对象
>>> A.bar()
Companion object called

>>> val sum = { x: Int, y: Int -> x + y } // 定义lambda表达式
>>> println(sum(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()
  }
}

>>> val list = listOf(1, 2, 3, 4)
>>> list.filter { it % 2 == 0 }
[2, 4]

>>> val list = listOf(1, 2, 3, 4)
>>> list.map { it * it }
[1, 4, 9, 16]

val canBeInClub27 = { p: Person -> p.age <= 27 }

>>> val people = listOf(Person("Alice", 27), Person("Bob", 31))
>>> println(people.all(canBeInClub27))
false

val canBeInClub27 = { p: Person -> p.age <= 27 }

>>> val people = listOf(Person("Alice", 27), Person("Bob", 31))
>>> println(people.any(canBeInClub27))
true

>>> val people = listOf(Person("Alice", 27), Person("Bob", 31))
>>> println(people.count(canBeInClub27))
1

>>> val people = listOf(Person("Alice", 27), Person("Bob", 31))
>>> println(people.find(canBeInClub27))
Person(name=Alice, age=27)