昨天的贴一下
我打算以十二生肖为包名,建立kotlin的入门demo库
//the options set pakcage
package mouse
/**
* @Author: zhangQi
* @Date: 2020-08-29 21:05
*/
/**
* this function opened to the package visible
* it takes a String array (type) as parameters named: args (call name)
*/
fun main(args: Array<String>) {
//the ; can not write behind ), it's different between java & kotlin
println("Hello World!")
}
package mouse
/**
* @Author: zhangQi
* @Date: 2020-08-29 21:15
*/
//this param is String called name , using val
class Greeter(val name:String){
fun greet(){
println("Hello,$name")
}
}
fun main(){
//create an object dont using "new" XXX
Greeter("World").greet()
}
/*为什么选择 Kotlin？
简洁: 大大减少样板代码的数量。
安全: 避免空指针异常等整个类的错误。
互操作性: 充分利用 JVM、Android 和浏览器的现有库。
工具友好: 可用任何 Java IDE 或者使用命令行构建。*/
package cow
/**
* @Author: zhangQi
* @Date: 2020-08-29 21:24
*/
/**
* the params a is a int, b is a int,return a int type
*/
fun sum(a:Int,b:Int):Int{
return a+b
}
/**
* functional fun the return type is automated
*/
fun sum1(a:Int,b:Int) = a+b
/**
* the public fun must be use return type :Int or some else
*/
public fun sum2(a:Int,b:Int):Int = a+b
/**
* Unit means void in java , no return type
*/
fun printSum(a:Int,b:Int):Unit{
print(a+b)
}
/**
* if return type is Unit,
* then can ignore write that,
* also the public fun
*/
public fun printSum2(a:Int,b:Int){
print(a+b)
}
fun main(){
println(sum(1,2))
println(sum1(3,4))
println(sum2(5,6))
printSum(1,3)
println()
printSum2(1,4)
}
package cow
/**
* @Author: zhangQi
* @Date: 2020-08-29 21:36
*/
/**
* It can be using 'vararg' set more parameters
* Just like the java (String... args)
*/
fun vars(vararg v:Int){
for(vt in v){
print(vt)
}
}
fun main(args: Array<String>){
vars(1,2,3,4,5,6)
//print 123456
}
package cow
/**
* @Author: zhangQi
* @Date: 2020-08-29 21:39
*/
//using lambda to do Math
fun main(){
/**
* val one sumLambda: get two params Int,Int -> then
* response Int type value, this value is the x,y->x+y make sum
* {x,y->x+y}
*/
val sumLambda: (Int,Int) -> Int = {x,y -> x+y}
/**
* so you can saw 3
*/
println(sumLambda(1,2))
}
package cow
/**
* @Author: zhangQi
* @Date: 2020-08-29 21:59
*/
fun variables(){
val a: Int = 1
val b = 1
//be declare then asset
var c: Int
c = 1
var x = 5
x += 1
println(a)
println(b)
println(c)
println(x)
}
fun main(){
variables()
/**
* 1
* 1
* 1
* 6
*/
}
package cow
/**
* @Author: zhangQi
* @Date: 2020-08-29 22:56
*/
//just like java
fun singleAnnotation(){
//single annotation
/*val hello:(String) -> Unit = {x->x+" ~~~"}
* the block annotation,it can be log kotlin.Unit */
val hello:(String) -> String = {x->x+" ~~~"}
print(hello("你好呀"))
}
fun main(){
singleAnnotation()
//single annotation,sysout: the variables "你好呀 ~~~"from lambda val
}
package cow
/**
* @Author: zhangQi
* @Date: 2020-08-29 23:06
*/
/**
* string template ,
* using 'replace' replacing that string template will create new one
*
*/
fun template(){
var a = 1
//simple name in template
val s1 = "a is $a"
a = 2
//any expression in template
val s2 = "${s1.replace("is","was")},but now is $a"
println(s1)
println(s2)
}
fun main(){
template()
}
package cow
/**
* @Author: zhangQi
* @Date: 2020-08-30 15:04
*/
fun checkNull(){
//the tyoe String and ? means the variable can be null
var age: String? = "23"
// var age: String? //if not initialize, the age!!.toInt() throws Error must be initialized
//throw exception
val ages = age!!.toInt()
//return null
val ages1 = age?.toInt()
//if age is null return -1
val ages2 = age?.toInt() ?: -1
println(age)
println(ages)
println(ages1)
println(ages2)
}
fun main(){
checkNull()
}
package cow
/**
* @Author: zhangQi
* @Date: 2020-08-30 15:04
*/
fun checkNull() {
//the tyoe String and ? means the variable can be null
var age: String? = "23"
// var age: String? //if not initialize, the age!!.toInt() throws Error must be initialized
//throw exception
val ages = age!!.toInt()
//return null
val ages1 = age?.toInt()
//if age is null return -1
val ages2 = age?.toInt() ?: -1
println(age)
println(ages)
println(ages1)
println(ages2)
}
fun main() {
checkNull()
var ele1 = "123"
var ele2 = "412"
//make a array using arrayOf(element,element1,...)
example1(arrayOf(ele1, ele2))
}
//two String params check and parseInt multiply
fun example1(params: Array<String>) {
if (params.size < 2) {
println("Two params expected")
return
}
//lambda string -> int
//toInt() : public actual inline fun String.toInt(): Int = java.lang.Integer.parseInt(this) using java function
val parseInt: (String) -> Int = { x -> x.toInt() }
val x = parseInt(params[0])
val y = parseInt(params[1])
if (x != null && y != null) {
println(x * y)
}
}
package cow
/**
* @Author: zhangQi
* @Date: 2020-08-30 15:58
*/
/**
* getStringLength, but the convert to String is automatic
*/
fun getStringLength(obj: Any): Int? {
//judgement the type of data : Any
if (obj is String) {
return obj.length
} else if (obj !is String) {
println("obj is not String")
} else if (obj is Int) {
println("obj is Int, get length of obj : Int")
return obj.length
} else {
println("other type don't return length~")
return null
}
return null
}
fun getStringLength1(obj: Any): Int? {
if(obj is String){
return obj.length
}
return null
}
fun getStringLength2(obj: Any):Int?{
if(obj !is String){
return null
}
return obj.length
}
fun main(args:Array<String>){
//变量永远不会被修改，可以使用“val”声明为不可变变量
val str1 :String = "www.cnblogs.com/ukzq"
val l1 = getStringLength(str1)
println(l1)
val l2 = getStringLength1(str1)
println(l2)
val l3 = getStringLength2(str1)
println(l3)
val test2 :Long = 20160404
getStringLength(test2)
getStringLength1(test2)
getStringLength2(test2)
}