kotlin历程
- 2011年7月，JetBrains推出Kotlin项目，这是一个面向JVM的新语言
- 2012年2月，JetBrains以Apache 2许可证开源此项目
- 2016年2月15日，Kotlin v1.0发布。这被认为是第一个官方稳定版本，并且JetBrains已准备从该版本开始的长期向后兼容性
- 2017年，Google宣布在Android上为Kotlin提供一等支持
kotlin的优势
- 比java 更快
如下：创建1000000 个对象
public class JPerson {public JPerson(String name, Integer age, Integer sex) {this.name = name;this.age = age;this.sex = sex;}private String name ;private Integer age;private Integer sex ;public String getName() {return name;}public void setName(String name) {this.name = name;}public Integer getAge() {return age;}public void setAge(Integer age) {this.age = age;}public Integer getSex() {return sex;}public void setSex(Integer sex) {this.sex = sex;}}
执行结果
public class ReflectDemo {public static void main(String[] args) throws InterruptedException {int i =0;long start = System.currentTimeMillis();while (i<1000000){JPerson jPerson = new JPerson("zhangsan",1,1);i++ ;}long end = System.currentTimeMillis();System.out.println("java本次耗时:"+(end-start));}}
data class KPerson(val name: String, val age: Int, val sex: Int)fun main(args: Array<String>) {var i = 0val start = System.currentTimeMillis()while (i < 1000000) {val kPerson = KPerson("张三", 1, 1)i = i.inc()}val end = System.currentTimeMillis()println("kotlin本次耗时:${(end - start)}")}执行结果
3. 比java 语法更简洁、下面是一些简单示例，对kotlin 语法不太理解没关系、后续会对kotlin有详细讲解文章
//Java
System.out.print("hello world");
System.out.println("hello world");
//Kotlin 省略 System.out
print("hello world")
println("hello world")//Java
String name = "hello world";
final String name = "hello world";
//Kotlin 自动类型推断
var name = "hello world"
val name = "hello world"//Java
if (text != null) {int length = text.length();
}
//Kotlin 省略if 判断以及括号，可以使用it代替text
text?.let {val length = it.length
}//Java
String firstName = "Android";
String lastName = "Architect";
String message = "My name is: " + firstName + " " + lastName;
//Kotlin 直接使用插值运算符
val firstName = "Android"
val lastName = "Architect"
val message = "My name is: $firstName $lastName"//Java
String text = "First Line\n" +"Second Line\n" +"Third Line";//Kotlin 写法更加优雅
val text = """|First Line|Second Line|Third Line""".trimMargin()//Java
Car car = (Car) object;
//Kotlin 使用as 转换 而且转换后后续无需再次转换
var car = object as Car
car.name // 可以直接使用car 属性//Java
int score = // some score;
String grade;
switch (score) {case 10:case 9:grade = "Excellent";break;case 8:case 7:case 6:grade = "Good";break;case 5:case 4:grade = "OK";break;case 3:case 2:case 1:grade = "Fail";break;default:grade = "Fail";
}//Kotlin
var score = // some score
var grade = when (score) {9, 10 -> "Excellent"in 6..8 -> "Good"4, 5 -> "OK"in 1..3 -> "Fail"else -> "Fail"
}// labamda表达式 Java 8+
cars.stream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));//Kotlin
cars.forEach {println(it.speed)
}cars.filter { it.speed > 100 }.forEach { println(it.speed)}//Java 静态方法，私有构造比较
public class Utils {private Utils() {// This utility class is not publicly instantiable}public static int getScore(int value) {return 2 * value;}}//Kotlin
class Utils private constructor() {companion object {fun getScore(value: Int): Int {return 2 * value}}
}
// java 对象声明
public class JPerson {public JPerson(String name, Integer age, Integer sex) {this.name = name;this.age = age;this.sex = sex;}private String name ;private Integer age;private Integer sex ;public String getName() {return name;}public void setName(String name) {this.name = name;}public Integer getAge() {return age;}public void setAge(Integer age) {this.age = age;}public Integer getSex() {return sex;}public void setSex(Integer sex) {this.sex = sex;}}// kotlin
data class KPerson(val name: String, val age: Int, val sex: Int)-
让它比Java更安全，能够静态检测常见的陷阱。如：引用空指针
-
更高级特性，比如运算符重载，扩展函数
运算符重载 对象可以-加减乘除了
data class Point(val x: Int, val y: Int) {operator fun plus(other: Point) = Point(x + other.x, y + other.y)operator fun plus(value: Int) = "toString: ${Point(x + value, y + value)}"
}fun main(args: Array<String>) {val p1 = Point(1, 2)val p2 = Point(3, 4)println(p1 + p2)println(p1 + 3)
}/*
Point(x=4, y=6)
toString: Point(x=4, y=5)
*/扩展函数，person 类没有order 这个函数，却可以在类的外部给其声明方法
6. 支持协程.让开发更简单
下面并行代码、如果串行执行需要5s，实际执行结果只需要2s、也就是单个任务最大耗时，并行代码比java 简单
@Testfun checkRunBlocking() {val start = System.currentTimeMillis()runBlocking {launch {println("launch1")delay(1000)
// Thread.sleep(1000)println("launch1 finished")}launch {println("launch2")delay(1000)
// Thread.sleep(1000)println("launch2 finished")}launch {println("launch2")delay(1000)
// Thread.sleep(1000)println("launch2 finished")}launch {println("launch2")delay(1000)
// Thread.sleep(1000)println("launch2 finished")}launch {println("launch2")delay(2000)
// Thread.sleep(1000)println("launch2 finished")}}
// Thread.sleep(1000)val end = System.currentTimeMillis()println("本次耗时：${end - start}")}
- Kotlin可以编译成Java字节码，也可以编译成JavaScript，前后端可公用一套模型 、后面会单独创建一个项目来讲解这块内容
仓库
https://gitcode.net/mid120/kotlin
总结
从java 转kotlin 刚开始觉得kotlin语法很怪异，比如每行最后不需要加分号，但总是莫名的加上，结果idea 提示是灰色，然后又把分号干掉。然后就是简单写法、很容易让代码看起来是声明方法，实际是调用方法。再加上项目压力，很容易写出一半是kotlin 一半是java 代码。但是时间长久之后，慢慢发现kotlin的简单写法，会干掉大部分java代码，公司项目重构就是用kotlin 替换java ，代码直接少了75%，相当给力。所以，kotlin还是非常优秀的语言，抓紧时间学习吧
