定义包的规范应位于源文件的顶部：
package com.web3 import java.util.* // ...
不需要匹配目录和包：源文件可以任意放在文件系统中。
具有返回Int类型的两个Int参数的函数：
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))
}
函数与表达主体和推断返回值的类型：
fun sum(a: Int, b: Int) = a + b
fun main(args: Array<String>) {
println("sum of 19 and 23 is ${sum(19, 23)}")
}
函数返回无意义值：
fun printSum(a: Int, b: Int): Unit {
println("sum of $a and $b is ${a + b}")
}
fun main(args: Array<String>) {
printSum(-1, 8)
}
Unit返回类型可以省略：
fun printSum(a: Int, b: Int) {
println("sum of $a and $b is ${a + b}")
}
fun main(args: Array<String>) {
printSum(-1, 8)
}
分配一次(只读)局部变量：
fun main(args: Array<String>) {
val a: Int = 1 // immediate assignment
val b = 2 // `Int` type is inferred
val c: Int // Type required when no initializer is provided
c = 3 // deferred assignment
println("a = $a, b = $b, c = $c")
}
可变变量：
fun main(args: Array<String>) {
var x = 5 // `Int` type is inferred
x += 1
println("x = $x")
}
Kotlin中的注释类似于Java和JavaScript，Kotlin支持行尾和块注释。
// This is an end-of-line comment /* This is a block comment on multiple lines. */
与Java不同的是，Kotlin中的块注释可以嵌套。
fun main(args: Array<String>) {
var a = 1
// simple name in template:
val s1 = "a is $a"
a = 2
// arbitrary expression in template:
val s2 = "${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)}")
}
使用if表达式：
fun maxOf(a: Int, b: Int) = if (a > b) a else b
fun main(args: Array<String>) {
println("max of 0 and 42 is ${maxOf(0, 42)}")
}
当可能为null值时，引用必须被明确地标记为可空(null)。
如果str不包含整数，则返回null：
fun parseInt(str: String): Int? {
// ...
}
使用可返回null值的函数：
fun parseInt(str: String): Int? {
return str.toIntOrNull()
}
fun printProduct(arg1: String, arg2: String) {
val x = parseInt(arg1)
val y = parseInt(arg2)
// Using `x * y` yields error because they may hold nulls.
if (x != null && y != null) {
// x and y are automatically cast to non-nullable after null check
println(x * y)
}
else {
println("either '$arg1' or '$arg2' is not a number")
}
}
fun main(args: Array<String>) {
printProduct("6", "7")
printProduct("a", "7")
printProduct("a", "b")
}
上面代码，也可以编写成下面 -
fun parseInt(str: String): Int? {
return str.toIntOrNull()
}
fun printProduct(arg1: String, arg2: String) {
val x = parseInt(arg1)
val y = parseInt(arg2)
// ...
if (x == null) {
println("Wrong number format in arg1: '${arg1}'")
return
}
if (y == null) {
println("Wrong number format in arg2: '${arg2}'")
return
}
// x and y are automatically cast to non-nullable after null check
println(x * y)
}
fun main(args: Array<String>) {
printProduct("6", "7")
printProduct("a", "7")
printProduct("99", "b")
}
is运算符检查表达式是否是类型的实例。 如果一个不可变的局部变量或属性是指定类型，则不需要显式转换：
fun getStringLength(obj: Any): Int? {
if (obj is String) {
// `obj` is automatically cast to `String` in this branch
return obj.length
}
// `obj` is still of type `Any` outside of the type-checked branch
return null
}
fun main(args: Array<String>) {
fun printLength(obj: Any) {
println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
}
printLength("Incomprehensibilities")
printLength(1000)
printLength(listOf(Any()))
}
上面代码，也可以编写成下面 -
fun getStringLength(obj: Any): Int? {
if (obj !is String) return null
// `obj` is automatically cast to `String` in this branch
return obj.length
}
fun main(args: Array<String>) {
fun printLength(obj: Any) {
println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
}
printLength("Incomprehensibilities")
printLength(1000)
printLength(listOf(Any()))
}
上面代码，还可以编写成下面 -
fun getStringLength(obj: Any): Int? {
// `obj` is automatically cast to `String` on the right-hand side of `&&`
if (obj is String && obj.length > 0) {
return obj.length
}
return null
}
fun main(args: Array<String>) {
fun printLength(obj: Any) {
println("'$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)
}
}
上面代码，可以编写成如下 -
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"))
}
使用in操作符检查数字是否在指定范围内：
fun main(args: Array<String>) {
val x = 10
val y = 9
if (x in 1..y+1) {
println("fits in range")
}
}
检查一个数字是否超出指定范围：
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")
}
}
上面代码，执行结果如下 -
-1 is out of range list size is out of valid list indices range too迭代范围：
fun main(args: Array<String>) {
for (x in 1..5) {
print(x)
}
}
或过程：
fun main(args: Array<String>) {
for (x in 1..10 step 2) {
println(x)
}
println("===============================")
for (x in 9 downTo 0 step 3) {
println(x)
}
}
上面代码，执行结果如下 -
1 3 5 7 9 =============================== 9 6 3 0
迭代集合：
fun main(args: Array<String>) {
val items = listOf("apple", "banana", "kiwi")
for (item in items) {
println(item)
}
}
检查集合是否包含一个对象，使用in运算符：
fun main(args: Array<String>) {
val items = setOf("apple", "banana", "kiwi")
when {
"orange" in items -> println("juicy")
"apple" in items -> println("apple is fine too")
}
}
使用lambda表达式过滤映射集合：
fun main(args: Array<String>) {
val fruits = listOf("banana", "avocado", "apple", "kiwi")
fruits
.filter { it.startsWith("a") }
.sortedBy { it }
.map { it.toUpperCase() }
.forEach { println(it) }
} 