Kotlin 基础语法详细介绍

package my.demo

import java.util.*

// ...

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

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

fun printSum(a: Int, b: Int): Unit {
 print(a + b)
}

fun printSum(a: Int, b: Int) {
 print(a + b)
}

val a: Int = 1
val b = 1  // `Int` 类型是被编译器推理出
val c: Int // 当变量的初始值没有被提供时，需要定义变量的类型
c = 1    // 赋值

var x = 5 // `Int` 类型是被编译器推理出的
x += 1

// 这是一段行注释

/* 这是一段代码块
  注释 */

fun main(args: Array<String>) {
 if (args.size == 0) return

 print("First argument: ${args[0]}")
}

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

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

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

fun main(args: Array<String>) {
 if (args.size < 2) {
  print("Two integers expected")
  return
 }

 val x = parseInt(args[0])
 val y = parseInt(args[1])

 // Using `x * y` yields error because they may hold nulls.
 if (x != null && y != null) {
  // x and y are automatically cast to non-nullable after null check
  print(x * y)
 }
}

// ...
 if (x == null) {
  print("Wrong number format in '${args[0]}'")
  return
 }
 if (y == null) {
  print("Wrong number format in '${args[1]}'")
  return
 }

 // x and y are automatically cast to non-nullable after null check
 print(x * y)

fun getStringLength(obj: Any): Int? {
 if (obj is String) {
  // `obj` is automatically cast to `String` in this branch
  return obj.length
 }

 // `obj` is still of type `Any` outside of the type-checked branch
 return null
}

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

 // `obj` is automatically cast to `String` in this branch
 return obj.length
}

fun getStringLength(obj: Any): Int? {
 // `obj` is automatically cast to `String` on the right-hand side of `&&`
 if (obj is String && obj.length > 0)
  return obj.length

 return null
}

fun main(args: Array<String>) {
 for (arg in args)
  print(arg)
}

for (i in args.indices)
 print(args[i])

fun main(args: Array<String>) {
 var i = 0
 while (i < args.size)
  print(args[i++])
}

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

if (x in 1..y-1)
 print("OK")

if (x !in 0..array.lastIndex)
 print("Out")

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

for (name in names)
 println(name)

if (text in names) // names.contains(text) is called
 print("Yes")

names
  .filter { it.startsWith("A") }
  .sortedBy { it }
  .map { it.toUpperCase() }
  .forEach { print(it) }