Kotlin 基本语法

定义包

package com.demo

定义方式和Java一样，没什么区别

import java.util.*

导入类也是一样

函数定义

定义sum函数，有Int两个参数，返回Int

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

疯狂写法(表达式)

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

有两个参数，没有返回

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

Unit关键字和java的void一样效果

当然也可以这样写:

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

定义变量

不变变量，如果尝试更改，则会报错

fun main() {
val a: Int = 1  //
val b = 2   // `Int` 自动判断类型
val c: Int  // 当没设置初始值的时候，需要写类型
c = 3       // 自动判断类型
println("a = $a, b = $b, c = $c")
}

可变变量(可更改变量)

var a=5
x+=1

注释

// 我是注释

/*我是注释2*/

字符串模板

var a = 1
//定义模板
val s1 = "a is $a"

a = 2
//使用模板进行替换
val s2 = "${s1.replace("is", "was")}, but now is $a"
println(s2)

条件语句

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

使用表达式:

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

null值和null值检测

//
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")
}
}

类型检查和自动转型

fun getStringLength(obj: Any): Int? {
if (obj is String) {
// 自动转型
return obj.length
}
return null
}

for 循环

fun forLoop(){
val items = listOf("aa", "bb", "cc");
for (item in items) {
println(item);
}
for (index in items.indices) {
println("item at $index is $items[index]");
}
}

while 循环

fun whileLoop(): Unit {
val whiles = listOf("app", "bana", "kk");
var index = 0
while (index < whiles.size) {
println("item at $index is ${whiles[index]}");
index++;
}
}

when 表达式

fun whenExpression(obj: Any): String =
when (obj) {
1 -> "One"
"hello" -> "Greeting"
is Long -> "Long"
is String -> "String"
else -> "unknown"
}

使用范围(ranges)

fun useRanges(): Unit {
val x = 10
val y = 9
if (x in 1..y + 1) {
println("fits in range")
}
//
if (-1 !in 0..y - 1) {
println("-1 not in range")
}
//
for (x in 1..5) {
print(x)
}
//
for (xx in 1..10 step 2) {
print(xx);
}

}

使用集合

fun useCollections(): Unit {
val items = setOf("apple", "banana", "kiwi")
when {
"orange" in items -> println("juicy")
"apple" in items -> println("apple is fine too")
}
}