Kotlin学习之基础语法

写在前面的：
1，Kotlin学习之基础语法
2，Kotlin学习之类和继承
3，Kotlin学习之属性和字段
4，Kotlin学习之接口，可见性修饰符

去年6月份开始写了第一篇Kotlin基础语法的文章，但后来由于项目繁重，加上项目本身没有用到Kotlin,所以没有写下去，现在开始重头记录Kotlin学习历程！！！

1，定义函数
定义有两个Int参数，返回为Int的函数

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

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

结果：sum of a+b is 8

将表达式作为函数体，返回值类型自动推断的函数

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

fun main(args : Array<String>){
println("sum of 5 and 6 is ${sum(5,6)}")
}

结果：sum of 5 and 6 is 11

函数无返回值

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

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

结果：sum of -1 and 8 is 7

函数无返回值则Unit可省略

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

fun main(args : Array<String>){
println(-1,8)
}
结果：sum of -1 and 8 is 7

2,定义变量
a:定义常量

fun main(args : Array<String>){
val a : Int = 1  // 立即赋值
val b = 2        // 自动推断出 `Int` 类型
val c : Int       // 如果没有初始值类型不能省略
c = 3             // 明确赋值
println("a=$a , b=$b , c=$c")
}
结果：a=1, b=2 , c=3

b:定义变量

var x= 5;
fun main(args : Array<String>){
x+=1;
println("x = $x")

}
结果：x = 6

3,使用字符串模板

fun main(args : Array<String>){
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

4,条件表达式

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

也可以这样写：：：
fun maxOf(a : Int，b : Int) = if(a > b) a else b

fun main(args : Array<String>){
println("maxOf 2 and 5 is ${maxOf(2，5)}")
}

结果:maxOf 2 and 5 is 5

5,空类型安全检测
当某个变量的值可以为 null 的时候，必须在声明处的类型后添加 ? 来标识该引用可为空。

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

}

fun printProduct(args1 : String，args2 : String){
var a= parseInt(args1)
var b= parseInt(args2)
if(a== null){
println("$a is not a number")
return
}

if(b== null){
println("$b is not a number")
return
}

 // 在空检测后，a和 b 会自动转换为非空值
println(a * b)
}

fun main(args : Array<String>){
println("5","6")
println("x","55")
println("99","y")
}

结果：30
           x is not a number
           y is not a number

6,使用类型检测及自动类型转换

fun printStringLength(obj : Any) : Int?{
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, not a string"} ")
}

printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}

结果:
'Incomprehensibilities' string length is 21 
'1000' string length is ... err, not a string 
'[java.lang.Object@1be6f5c3]' string length is ... err, not a string 

7,使用for循环语句

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

}
}
结果:
apple
bear
banana

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

}
结果：
item at 0 is apple
item at 1 is bear
item at 2 is banana
}

8,使用 while 循环

fun main(args: Array<String>) {
val items= listOf("apple"，"bear"，"banana")
var index= 0
while(index <items.size){
println("item at $item is ${items[item]}")
index++
}
结果：
item at 0 is apple
item at 1 is bear
item at 2 is banana

9,使用 when 表达式

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"))
}

结果：
One
Greeting
Long
Not a string
Unknown

10， 使用区间（range）
a:使用 in 运算符来检测某个数字是否在指定区间内：

fun main(args : Array<String>){
val a= 10
val b= 9
if(a in 1.. b+1){
println("a in b")
}

}

结果：a in b

fun main(args : Array<String>){
val lists= listOf("a"，"b"，"c")
if(-1 !in 0.. lists.lastIndex){
println("-1 is out of range")
}

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

b:区间迭代:

fun main(args : Array){
for (x in 1.. 5){
print(x)
}
}
结果：12345

c:数列迭代：

fun main(args: Array<String>) {
    for (x in 1..10 step 2) {
        print(x)
    }
    println()
    for (x in 9 downTo 0 step 3) {
        print(x)
    }
}
结果：
13579
9630

d:使用 in 运算符来判断集合内是否包含某实例：

fun main(args : Array<String>){
val items= listOf("apple", "banana", "kiwifruit")
when{
"orange" in items -> println("juicy")
"apple" in items -> println("apple is fine too")
}
}
结果：apple is fine too

11,使用 lambda 表达式来过滤（filter）和映射（map）集合：

fun main(args : Array<String>){
val list= listOf("banana", "avocado", "apple", "kiwifruit")
list
.filter{it.startWith("a")}//过滤以字母a开头的元素
.sortedBy{it}//对过滤后的元素进行排序
.map{it.toUpperCase()}//将拍完序的元素全部转换为大写字母
.forEach{println(it)}//打印元素
}
结果：
APPLE
AVOCADO

12,创建基本类及其实例(有待消化，暂时先挖坑)：

fun main(args: Array<String>) {
    val rectangle = Rectangle(5.0, 2.0) // 不需要“new”关键字
    val triangle = Triangle(3.0, 4.0, 5.0)
    println("Area of rectangle is ${rectangle.calculateArea()}, its perimeter is ${rectangle.perimeter}")
    println("Area of triangle is ${triangle.calculateArea()}, its perimeter is ${triangle.perimeter}")
}

abstract class Shape(val sides: List<Double>) {
    val perimeter: Double get() = sides.sum()
    abstract fun calculateArea(): Double
}

interface RectangleProperties {
    val isSquare: Boolean
}

class Rectangle(
    var height: Double,
    var length: Double
) : Shape(listOf(height, length, height, length)), RectangleProperties {
    override val isSquare: Boolean get() = length == height
    override fun calculateArea(): Double = height * length
}

class Triangle(
    var sideA: Double,
    var sideB: Double,
    var sideC: Double
) : Shape(listOf(sideA, sideB, sideC)) {
    override fun calculateArea(): Double {
        val s = perimeter / 2
        return Math.sqrt(s * (s - sideA) * (s - sideB) * (s - sideC))
    }
}
结果：
Area of rectangle is 10.0, its perimeter is 14.0
Area of triangle is 6.0, its perimeter is 12.0