快速上⼿ Kotlin

基础语法

var age: Int = 19
val name: String = "Tom"
var age = 19
val name = "Tom"

String?
String
string!!

var name: String = "Tom"
var name2: String? = "Tom"

name = name2   // name2大于name   不能这样赋值
name = name2!!

name2 = name


var name: String? = null

println(name?.length)

fun printLength(name: String){

    println(name.length)

}

printLength(null)

函数基础语法

fun echo(name: String): String? {

    println("$name")
    return name
}

与 Java 代码互调

Java 与 Kotlin 交互的语法变化

// Utils.kt
fun echo(name: String) {
    println("$name")
}

// Main.java
public static void main(String[] args) {
    UtilsKt.echo("hello");
}

Java 与 Kotlin 交互的语法变化

object Test {
    fun sayMessage(msg: String){
        println(msg)
    }
}

Test.INSTANCE.sayMessage("hello");  //kotlin code

Test.sayMessage("hello")     //java code


TestMain.class  //java code

TestMain::class.java  //kotlin code 

Kotlin 没有封装类

//java code
public interface AInterface {
    void putNumber(int num);
    void putNumber(Integer num);
}

//kotlin code
a.putNumber(100)

Kotlin 类型空值敏感

//kotlin code
fun sayMessage(msg: String) {
    println(msg)
}

//java code
a.sayMessage(null)


//java code
String format(String str) {
    return str.isEmpty() ? null : str;
}

//kotlin code
fun function(str: String) {
    val fmt1 = format(str)
    val fmt2:String = format(str)
    val fmt3:String? = format(str)
}

Kotlin 没有静态变量与静态⽅法

object Test {
    fun sayMessage(msg: String){
        println(msg)
    }
}


Test.INSTANCE.sayMessage("hello")

Test.sayMessage("hello")


object Test {
    @JvmStatic
    fun sayMessage(msg: String){
        println(msg)
    }
}