kotlin语法
1. 程序入口
跟Java里面main函数一样，Koltin的入口也是main函数
fun main() {
println("Hello world!")
}
另外一个接收参数的main函数
fun main() {
prinln("Hi kotlin")
}
2. 函数声明
跟Java 函数参数里面 先声明类型，再声明变量不一样，Kotlin的函数里面参数是先声明变量，再指定类型
例如，一个求和函数声明如下：
fun sum(a:Int, b:Int):Int{
return a+b
}
另外，返回值类型也是跟着函数名字后面，用：分隔。还有一个细节不知道注意没有，Kotlin语法里结尾是不需要分号的。爱惜键盘按键。
甚至，可以直接写成
fun sum(a:Int, b:Int) = a+b
3. 没有返回值的函数声明 Unit
fun printSum(a:Int, b:Int):Unit {
println("sum of $a and $b is ${a+b}")
}
4.变量声明
常量声明，一旦用了val，就无法修改改变量，类似Java中final
val a : Int = 1;
val b = 2
val c : Int
c = 3
变量声明 var
var x = 4
x = 5
x += 1
5.类和实例
类声明 class
例如:
class Rectangle(var height:Double, var width:Double) {
var perimeter = (height + width) * 2
}
6.继承
一个类可以被继承需要用关键字open 修饰
open class Shape
class Rectangle(var height:Double, var width:Double):Shape(){
var perimeter = (height + width) * 2
}
7.字符串
fun main() {
var a = 1
val S1 = "a is $a"
a = 2
val s2 = "${s1.replace("is", "was")}, but now is $a"
println(s2)
}
结果：a was 1, but now is 2
8.条件语句
fun maxOf(a:Int, b:Int) {
if(a > b) {
return a
} else {
return b
}
}
或者是
fun maxOf(a:Int, b:Int) = if (a > b) a else b
fun main(){
println("max of 0 and 42 is ${maxOf(0,42)}")
}
9. for 循环
val items = listOf("apple", "banana", "kiwifruit")
for (item in items) {
println(item)
}
val items = listOf("apple", "banana", "kiwifruit")
for (index in items.indices) {
println("item at $index is ${items[index]}")
}
10，while 循环
val items = listOf("apple", "banana", "kiwifruit")
var index = 0
while (index < items.size) {
println("item at $index is ${items[index]}")
index++
}
11.when
类似于Java里面switch
fun describe(obj: Any): String =
when (obj) {
1 -> "One"
"Hello" -> "Greeting"
is Long -> "Long"
!is String -> "Not a string"
else -> "Unknown"
}
12.范围Ranges
val x = 10
val y = 9
if (x in 1..y+1) {
println("fits in range")
}
13.集合
fun main() {
val items = listOf("apple", "banana", "kiwifruit")
for (item in items) {
println(item)
}
}
14.Nullable 和空值检测
fun parseInt(str:String):Int? {
//...
}
例如：
fun printProduct(arg1: String, arg2: String) {
val x = parseInt(arg1)
val y = parseInt(arg2)
// Using `x * y` yields error because they may hold nulls.
if (x != null && y != null) {
// x and y are automatically cast to non-nullable after null check
println(x * y)
}
else {
println("'$arg1' or '$arg2' is not a number")
}
}
15.类型检测 is
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
}
或者
fun getStringLength(obj: Any): Int? {
if (obj !is String) return null
// `obj` is automatically cast to `String` in this branch
return obj.length
}
又或者
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
}
More
Kotlin官网
Kotlin GitHub地址
The Kotlin Blog
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