kotlin 官方学习教程之基础语法详解

package my.demo

import java.util.*

// ...

fun sum(a: Int, b: Int): Int {
  return a + b
}

fun main(args: Array<String>) {
  print("sum of 3 and 5 is ")
  println(sum(3, 5))
}

fun sum(a: Int, b: Int) = a + b

fun main(args: Array<String>) {
  println("sum of 19 and 23 is ${sum(19, 23)}")
}

fun printSum(a: Int, b: Int): Unit {
  println("sum of $a and $b is ${a + b}")
}

fun main(args: Array<String>) {
  printSum(-1, 8)
}

fun printSum(a: Int, b: Int) {
  println("sum of $a and $b is ${a + b}")
}

fun main(args: Array<String>) {
  printSum(-1, 8)
}

fun main(args: Array<String>) {
  val a: Int = 1 // 定义变量时进行赋值
  val b = 2  // 自动推测变量类型为 Int
  val c: Int // 不提供初始化时需要定义变量类型
  c = 3    // 定义变量后再赋值

  println("a = $a, b = $b, c = $c")
}

fun main(args: Array<String>) {
  var x = 5 // 推断变量类型为 Int
  x += 1
  println("x = $x")
}

像 Java 和 JavaScript 一样，Kotlin 支持行注释和块注释。
// 这是行注释

/* 这是
  块注释 */

fun main(args: Array<String>) {
  var a = 1
   // 使用变量名作为模板:
  val s1 = "a is $a" 

  a = 2
   // 使用任意表达式作为模板:
  val s2 = "${s1.replace("is", "was")}, but now is $a"
  println(s2)
}

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

fun main(args: Array<String>) {
  println("max of 0 and 42 is ${maxOf(0, 42)}")
}

fun maxOf(a: Int, b: Int) = if (a > b) a else b

fun main(args: Array<String>) {
  println("max of 0 and 42 is ${maxOf(0, 42)}")
}

返回 NULL 如果 STR 不持有整数
fun parseInt(str: String): Int? {
  // ...
}

fun parseInt(str: String): Int? {
  return str.toIntOrNull()
}

fun printProduct(arg1: String, arg2: String) {
  val x = parseInt(arg1)
  val y = parseInt(arg2)

  // 使" X×Y "产生错误的因为他们可能有空值。
  if (x != null && y != null) {
    // 经过空值检测后，X 和 Y 自动转换为非空值。
    println(x * y)
  }
  else {
    println("either '$arg1' or '$arg2' is not a number")
  }  
}

 

fun main(args: Array<String>) {
  printProduct("6", "7")
  printProduct("a", "7")
  printProduct("a", "b")
}

fun parseInt(str: String): Int? {
  return str.toIntOrNull()
}

fun printProduct(arg1: String, arg2: String) {
  val x = parseInt(arg1)
  val y = parseInt(arg2)

  // ...
  if (x == null) {
    println("Wrong number format in arg1: '${arg1}'")
    return
  }
  if (y == null) {
    println("Wrong number format in arg2: '${arg2}'")
    return
  }

  // 经过空值检测后，X 和 Y 自动转换为非空值。
  println(x * y)
}

fun main(args: Array<String>) {
  printProduct("6", "7")
  printProduct("a", "7")
  printProduct("99", "b")
}

fun getStringLength(obj: Any): Int? {
  if (obj is String) {
    // obj 将会在这个分支被自动转换为 String 类型
    return obj.length
  }

  // obj 在种类检查外仍然是 Any 类型
  return null
}


fun main(args: Array<String>) {
  fun printLength(obj: Any) {
    println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
  }
  printLength("Incomprehensibilities")
  printLength(1000)
  printLength(listOf(Any()))
}

fun getStringLength(obj: Any): Int? {
  if (obj !is String) return null

  // obj 将会在这个分支被自动转换为 String 类型
  return obj.length
}


fun main(args: Array<String>) {
  fun printLength(obj: Any) {
    println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
  }
  printLength("Incomprehensibilities")
  printLength(1000)
  printLength(listOf(Any()))
}

fun getStringLength(obj: Any): Int? {
  // // obj 将会在 && 右边被自动转换为 String 类型
  if (obj is String && obj.length > 0) {
    return obj.length
  }

  return null
}


fun main(args: Array<String>) {
  fun printLength(obj: Any) {
    println("'$obj' string length is ${getStringLength(obj) ?: "... err, is empty or not a string at all"} ")
  }
  printLength("Incomprehensibilities")
  printLength("")
  printLength(1000)
}

fun main(args: Array<String>) {
  val items = listOf("apple", "banana", "kiwi")
  for (item in items) {
    println(item)
  }
}

fun main(args: Array<String>) {
  val items = listOf("apple", "banana", "kiwi")
  for (index in items.indices) {
    println("item at $index is ${items[index]}")
  }
}

fun main(args: Array<String>) {
  val items = listOf("apple", "banana", "kiwi")
  var index = 0
  while (index < items.size) {
    println("item at $index is ${items[index]}")
    index++
  }
}

fun describe(obj: Any): String =
when (obj) {
  1     -> "One"
  "Hello"  -> "Greeting"
  is Long  -> "Long"
  !is String -> "Not a string"
  else    -> "Unknown"
}

fun main(args: Array<String>) {
  println(describe(1))
  println(describe("Hello"))
  println(describe(1000L))
  println(describe(2))
  println(describe("other"))
}

fun main(args: Array<String>) {
  val x = 10
  val y = 9
  if (x in 1..y+1) {
    println("fits in range")
  }
}

fun main(args: Array<String>) {
  val list = listOf("a", "b", "c")

  if (-1 !in 0..list.lastIndex) {
    println("-1 is out of range")
  }
  if (list.size !in list.indices) {
    println("list size is out of valid list indices range too")
  }
}

for (x in 1..5) {
  print(x)
}

for (x in 1..10 step 2) {
  print(x)
}
for (x in 9 downTo 0 step 3) {
  print(x)
}

for (item in items) {
  println(item)
}

when {
  "orange" in items -> println("juicy")
  "apple" in items -> println("apple is fine too")
}

fruits
.filter { it.startsWith("a") }
.sortedBy { it }
.map { it.toUpperCase() }
.forEach { println(it) }